5.1 EviDEncE for Evolution
Daa-based qess: Missing links The discovery o ossils that ll in these gaps is
particularly exciting or biologists.
An objection to ossil evidence or evolution has
been gaps in the record, called missing links, 1 C alculate the length o Dilong paradoxus,
or example a link between reptiles and birds.
(a) (b) rom its head to the tip o its tail. [2]
(d) (c)
( g) 2 D educe three similarities between Dilong
paradoxus and reptiles that live on
Earth today. [3]
(h) (i) 3 Suggest a unction or the protoeathers o
(e) (f) 100 mm
Dilong paradoxus. [1 ]
(j)
4 Suggest two eatures which Dilong paradoxus
would have had to evolve to become
Figure 3 Drawings o ossils recently ound in Western capable o fight. [2]
China. They show Dilong paradoxus, a 130-million-year-old
tyrannosauroid dinosaur with protoeathers. ad: bones o 5 Explain why it is not possible to be certain
skull; e: teeth; g: tail vertebrae with protoeathers; hj: whether the protoeathers o Dilong paradoxus
limb bones are homologous with the eathers o birds. [2]
Evidence from selective breeding
Selective breeding o domesticated animals shows that
artifcial selection can cause evolution.
Humans have deliberately bred and used particular animal species or
thousands o years. I modern breeds o livestock are compared with
the wild species that they most resemble, the dierences are oten huge.
Consider the dierences between modern egg-laying hens and the
jungleowl o Southern Asia, or between Belgian Blue cattle and the aurochs
o Western Asia. There are also many dierent breeds o sheep, cattle and
other domesticated livestock, with much variation between breeds.
It is clear that domesticated breeds have not always existed in their
current orm. The only credible explanation is that the change has been
achieved simply by repeatedly selecting or and breeding the individuals
most suited to human uses. This process is called articial selection.
The eectiveness o articial selection is shown by the considerable changes
that have occurred in domesticated animals over periods o time that are
very short, in comparison to geological time. It shows that selection can
cause evolution, but it does not prove that evolution o species has actually
occurred naturally, or that the mechanism or evolution is natural selection.
Figure 4 Over the last 15,000 years many breeds o dog have been developed by artifcial 243
selection rom domesticated wolves
5 Evolution and biodivErsity
Homology and Data-based questions: Domestication of corn
evolution A wild grass called teosinte that grows in Central America was
probably the ancestor o cultivated corn, Zea mays. When teosinte
Looking or patterns, trends is grown as a crop, it gives yields o about 1 50 kg per hectare. This
and disrepanies: there are compares with a world average yield o corn o 4,1 00 kg per hectare
ommon eatures in the one at the start o the 2 1 st century. Table 1 gives the lengths o some cobs.
struture o verterate lims Corn was domesticated at least 7,000 years ago.
despite their varied use.
1 Calculate the percentage dierence in length between teosinte
Vertebrate limbs are used in
many dierent ways, such as and Silver Queen. [2]
walking, running, jumping, fying,
swimming, grasping and digging. 2 Calculate the percentage dierence in yield between teosinte
These varied uses require joints that
articulate in dierent ways, dierent and world average yields o corn. [2]
velocities o movement and also
dierent amounts o orce. It would 3 Suggest actors apart rom cob length, selected or by armers. [3]
be reasonable to expect them to
have very dierent bone structure, 4 Explain why improvement slows down over generations o
but there are in act common
eatures o bone structure that are selection. [3]
ound in all vertebrate limbs.
corn variety and origin length of ob (mm)
Patterns like this require Teosinte wild relative o orn 14
explanation. The only reasonable Early primitive orn rom Colomia 45
explanation so ar proposed in this Peruvian anient orn rom 500 bc 65
case is evolution rom a common Imriado primitive orn rom Colomia 90
ancestor. As a consequence, Silver Queen modern sweetorn 170
the common bone structure o
vertebrate limbs has become a classic Table 1
piece o evidence or evolution.
Figure 5 Corn cobs
Evidence from homologous structures
Evolution o homologous strutures y adaptive
radiation explains similarities in struture when there are
diferenes in untion.
D arwin pointed out in The Origin of Species that some similarities in
structure between organisms are supercial, or example between a
dugong and a whale, or between a whale and a sh. Similarities like
those between the tail ns o whales and shes are known as analogous
structures. When we study them closely we nd that these structures
are very dierent. An evolutionary interpretation is that they have had
244
5.1 EviDEncE for Evolution
dierent origins and have become similar because they perorm the
same or a similar unction. This is called convergent evolution.
Homologous structures are the converse o this. They are structures that
may look supercially dierent and perorm a dierent unction, but
which have what Darwin called a unity o type. He gave the example
o the orelimbs o a human, mole, horse, porpoise and bat and asked
what could be more curious than to nd that they include the same
bones, in the same relative positions, despite on the surace appearing
completely dierent. The evolutionary explanation is that they have
had the same origin, rom an ancestor that had a pentadactyl or ve-
digit limb, and that they have become dierent because they perorm
dierent unctions. This is called adaptive radiation.
There are many examples o homologous structures. They do not prove
that organisms have evolved or had common ancestry and do not reveal
anything about the mechanism o evolution, but they are dicult to
explain without evolution. Particularly interesting are the structures that
Darwin called rudimentary organs reduced structures that serve no
unction. They are now called vestigial organs and examples o them are
the beginnings o teeth ound in embryo baleen whales, despite adults
being toothless, the small pelvis and thigh bone ound in the body wall
o whales and some snakes, and o course the appendix in humans.
These structures are easily explained by evolution as structures that no
longer have a unction and so are being gradually lost.
Pentadactyl limbs
Comparison o the pentadactyl limb o mammals, birds, amphibians and reptiles
with dierent methods o locomotion.
The pentadactyl limb consists o these structures: classes that have limbs: amphibians, reptiles,
birds and mammals. Each o them has
Be se femb Hdmb pentadactyl limbs:
single bone in the humerus emur crocodiles walk or crawl on land and use their
proximal part webbed hind limbs or swimming
two bones in the radius and ulna tibia and fbula penguins use their hind limbs or walking and
distal part their orelimbs as fippers or swimming
group o wrist/ carpals tarsals echidnas use all our limbs or walking and
ankle bones also use their orelimbs or digging
series o bones in metacarpals and metatarsals rogs use all our limbs or walking and their
hindlimbs or jumping.
each o fve digits phalanges and phalanges
Dierences can be seen in the relative lengths and
The pattern o bones or a modication o it is thicknesses o the bones. Some metacarpals and
present in all amphibians, reptiles, birds and phalanges have been lost during the evolution o
mammals, whatever the unction o their limbs. the penguins orelimb.
The photos in gure 6 show the skeletons o
one example o each o the our vertebrates
245
5 Evolution and biodivErsity
Activity
Pentadactyl limbs in
mammals
mole
horse
porpoise Figure 6
bat speciation
human
Populations o a species can gradually diverge into
Figure 7 Pentadactyl limbs separate species by evolution.
(not to scale)
If two populations of a species become separated so that they do
Choose a colour code or not interbreed and natural selection then acts differently on the two
the types o bone in a populations, they will evolve in different ways. The characteristics of
pentadactyl limb and colour the two populations will gradually diverge. After a time they will be
the diagrams in fgure 7 to recognizably different. If the populations subsequently merge and have
show the type o each bone. the chance of interbreeding, but do not actually interbreed, it would be
How is each limb used? clear that they have evolved into separate species. This process is called
What eatures o the bones speciation.
in each limb make them well
adapted to the use? Speciation often occurs after a population of a species extends its range
by migrating to an island. This explains the large numbers of endemic
species on islands. An endemic species is one that is found only in a
certain geographical area. The lava lizards of the Galpagos Islands
are an example of this. One species is present on all the main islands
of the archipelago. On six smaller islands there is a closely related but
different species, formed by migration to the island and by subsequent
divergence.
246
5.1 EviDEncE for Evolution
Evidence from patterns of variation Pinta
Continuous variation across the geographical Genovesa
range o related populations matches the
concept o gradual divergence. Marchena
Santiago
I populations gradually diverge over time to become separate Fernandina Santa Cruz San Cristbal
species, then at any one moment we would expect to be able
to nd examples o all stages o divergence. This is indeed Santa Fe
what we nd in nature, as Charles Darwin describes in
C hapter II o The Origin of Species. He wrote: Isabela Santa Maria E s p a o la
Many years ago, when comparing, and seeing others compare, key T. delanonis T. habelii T. grayii
the birds from the separate islands ofthe Galpagos Archipelago, T. albemarlensis T. pacicus T. bivittatus
both one with another, and with those from the American T. duncanensis
mainland, I was much struck how entirely vague and arbitrary
is the distinction between species and varieties. Figure 8 Distribution of lava lizards in the
Galpagos Islands
Darwin gave examples o populations that are recognizably
dierent, but not to the extent that they are clearly separate
species. One o his examples is the red grouse o Britain and the willow
ptarmigan o Norway. They have sometimes been classied as separate
species and sometimes as varieties o the species Lagopus lagopus. This is a
common problem or biologists who name and classiy living organisms.
Because species can gradually diverge over long periods o time and
there is no sudden switch rom being two populations o one species to
being two separate species, the decision to lump populations together or
split them into separate species remains rather arbitrary.
The continuous range in variation between populations does not match TOK
either the belie that species were created as distinct types o organism
and thereore should be constant across their geographic range or that t wha exe a mpe mdes
species are unchanging. Instead it provides evidence or the evolution o be sed es hees?
species and the origin o new species by evolution.
Industrial melanism The useulness o a theory is
the degree to which it explains
Development o melanistic insects in polluted areas. phenomenon and the degree to
which it allows predictions to be
Dark varieties o typically light-coloured insects are called melanistic. made. One way to test the theory
The most amous example o an insect with a melanistic variety o evolution by natural selection is
is Biston betularia, the peppered moth. It has been widely used as through the use o computer models.
an example o natural selection, as the melanistic variety became The Blind Watchmaker computer
commoner in polluted industrial areas where it is better camoufaged model is used to demonstrate how
than the pale peppered variety. A simple explanation o industrial complexity can evolve rom simple
melanism is this: orms through artifcial selection. The
Weasel computer model is used to
Adult Biston betularia moths fy at night to try to nd a mate demonstrate how artifcial selection
and reproduce. can increase the pace o evolution
over random events. What eatures
During the day they roost on the branches o trees. would a computer model have to
include or it to simulate evolution by
Birds and other animals that hunt in daylight predate moths i natural selection realistically?
they nd them.
247
5 Evolution and biodivErsity
In unpolluted areas tree branches are covered in pale-coloured
lichens and peppered moths are well camoufaged against them.
Sulphur dioxide pollution kills lichens. Soot rom coal burning
blackens tree branches.
Melanic moths are well camoufaged against dark tree branches in
polluted areas.
In polluted areas the melanic variety o Biston betularia replaced
the peppered variety over a relatively short time, but not in non-
polluted areas.
Figure 9 Museum specimen of the Figure 10 The ladybug Adalia bipunctata
peppered form of Biston betularia has a melanic form which has become
mounted on tree bark with lichens common in polluted areas. A melanic male
from an unpolluted area is mating with a normal female here
Biologists have used industrial melanism as a classic example o
evolution by natural selection. Perhaps because o this, research
ndings have been repeatedly attacked. The design o some early
experiments into camoufage and predation o the moths has been
criticized and this has been used to cast doubt over whether natural
selection ever actually occurs.
Michael Majerus gives a careul evaluation o evidence about the
development o melanism in Biston betularia and other species o moth
in his book in the New Naturalist series (Moths, Michael Majerus,
HarperCollins 2002) . His nding is that the evidence or industrial
pollution causing melanism in Biston betularia and other species o moth is
strong, though actors other than camoufage can also infuence survival
rates o pale and melanic varieties.
Data-based questions: Predation rates in Biston betularia
One o the criticisms o the original experiments orms ( ty o each) o Biston betularia were
into predation o Biston betularia was that the placed in exposed positions on tree trunks and 50
moths were placed in exposed positions on tree millimetres below a joint between a major branch
trunks and that this is not normally where they and the tree trunk. This procedure was carried out
roost. The moths were able to move to more at two oak woods, one in an unpolluted area o
suitable positions but even so the criticisms have the New Forest in southern England and another
persisted on some websites. Experiments done in in a polluted area near Stoke-on-Trent in the
the 1 980s tested the eect o the position in which Midlands. The box plots in gure 1 1 show the
the moths were placed. Peppered and melanic percentage o moths eaten and moths surviving.
248
5.2 n AturAl sElEction
1 a) Deduce, with a reason from the data, peppered Stoke on Trent and New Forest
whether the moths were more likely to be
eaten if they were placed on the exposed New Forest/melanic/BJ 60 40
trunk or below the junction of a main New Forest/melanic/ET 38 62
branch and the trunk. [2] New Forest/peppered/BJ 74 26
b) Suggest a reason for the difference. [1 ] New Forest/peppered/ET 68 32
2 a) Compare and contrast the survival S t o ke /m e l a n i c /B J 72 28
rates of peppered and melanic moths
in the New Forest. [3] Stoke/melanic/ET 60 40
b) Explain the difference in survival Stoke/peppered/BJ 50 50
rate between the two varieties in the S t o ke /p e p p e re d /E T 42 58
New Forest. [3] melanic 0% 20% 40% 60% 80% 100%
3 D istinguish between the S toke- on- Trent and
New Forest woodlands in relative survival key
rates of peppered and melanic moths. [2] not eaten eaten
4 Pollution due to industry has decreased ET = exposed trunk BJ = branch junction
greatly near S toke- on- Trent since the 1 980s. Figure 11
Predict the consequences of this change for Source: Howlett and Majerus (1987) The Understanding of
industrial melanism in the peppered moth (Biston betularia)
Biston betularia. [4] Biol. J.Linn.Soc. 30, 3144
5.2 naa ee
uderstdig applictios
Natural selection can only occur i there is Changes in beaks o fnches on Daphne Major.
variation amongst members o the same species. Evolution o antibiotic resistance in bacteria.
Mutation, meiosis and sexual reproduction ntre of sciece
cause variation between individuals in a species.
Use theories to explain natural phenomena:
Adaptations are characteristics that make an the theory o evolution by natural selection
individual suited to its environment and way olie. can explain the development o antibiotic
resistance in bacteria.
Species tend to produce more ospring than
the environment can support.
Individuals that are better adapted tend to survive
and produce more ospring while the less well
adapted tend to die or produce ewer ospring.
Individuals that reproduce pass on
characteristics to their ospring.
Natural selection increases the requency o
characteristics that make individuals better
adapted and decreases the requency o other
characteristics leading to changes within the
species.
249
5 Evolution and biodivErsity
Figure 1 Populations o bluebells (Hyacinthoides vrition
non-scripta) mostly have blue fowers but
white-fowered plants sometimes occur Natural selection can only occur if there is variation
amongst members of the same species.
Charles Darwin developed his understanding of the mechanism that
causes evolution over many years, after returning to England from
his voyage around the world on HMS Beagle. He probably developed
the theory of natural selection in the late 1 830s, but then worked
to accumulate evidence for it. D arwin published his great work, The
Origin of Species, in 1 85 9. In this book of nearly 5 00 pages, he explains
his theory and presents the evidence for it that he had found over the
previous 20 to 30 years.
One of the observations on which Darwin based the theory of evolution
by natural selection is variation. Typical populations vary in many
respects. Variation in human populations is obvious height, skin colour,
blood group and many other features. With other species the variation
may not be so immediately obvious but careful observation shows that
it is there. Natural selection depends on variation within populations if
all individuals in a population were identical, there would be no way of
some individuals being favoured more than others.
Figure 2 Dandelions (Taraxacum ofcinale) source of rition
appear to be reproducing sexually when they
disperse their seed but the embryos in the Mutation, meiosis and sexual reproduction cause
seeds have been produced asexually so are variation between individuals in a species.
genetically identical
The causes of variation in populations are now well understood:
1 Mutation is the original source of variation. New alleles are produced
by gene mutation, which enlarges the gene pool of a population.
2 Meiosis produces new combinations of alleles by breaking up the
existing combination in a diploid cell. Every cell produced by meiosis
in an individual is likely to carry a different combination of alleles,
because of crossing over and the independent orientation of bivalents.
3 Sexual reproduction involves the fusion of male and female gametes.
The gametes usually come from different parents, so the offspring has
a combination of alleles from two individuals. This allows mutations
that occurred in different individuals to be brought together.
In species that do not carry out sexual reproduction the only source
of variation is mutation. It is generally assumed that such species will
not generate enough variation to be able to evolve quickly enough for
survival during times of environmental change.
adpttion
Adaptations are characteristics that make an individual
suited to its environment and way of life.
One of the recurring themes in biology is the close relationship between
structure and function. For example, the structure of a birds beak is
correlated with its diet and method of feeding. The thick coat of a musk
250
5.2 n AturAl sElEction
ox is obviously correlated with the low temperatures in its northerly Avy
habitats. The water storage tissue in the stem o a cactus is related to
inrequent rainall in desert habitats. In biology characteristics such as Adapa f bd beak
these that make an individual suited to its environment or way o lie
are called adaptations. The our photographs o
birds show the beaks o a
The term adaptation implies that characteristics develop over time heron, macaw, hawk and
and thus that species evolve. It is important not to imply purpose in woodpecker. To what diet
this process. According to evolutionary theory adaptations develop by and method o eeding is
natural selection, not with the direct purpose o making an individual each adapted?
suited to its environment. They do not develop during the lietime o
one individual. Characteristics that do develop during a lietime are
known as acquired characteristics and a widely accepted theory is that
acquired characteristics cannot be inherited.
Overproduction o ofspring Figure 3
Species tend to produce more ofspring than the
environment can support.
Living organisms vary in the number o ospring they produce.
An example o a species with a relatively slow breeding rate is the
southern ground hornbill, Bucorvus leadbeateri. It raises one fedgling
every three years on average and needs the cooperation o at least two
other adults to do this. However they can live or as long as 70 years
so in their lietime a pair could theoretically raise twenty ospring.
Most species have a aster breeding rate. For example, the coconut palm,
Cocos nucifera usually produces between 2 0 and 60 coconuts per year.
Apart rom bacteria, the astest breeding rate o all may be in the ungus
Calvatia gigantea. It produces a huge ruiting body called a giant puball
in which there can be as many as 7 trillion spores (7,000,000,000,000) .
Despite the huge variation in
breeding rate, there is an overall
trend in living organisms or more
ospring to be produced than the
environment can support. Darwin
pointed out that this will tend to
lead to a struggle or existence
within a population. There will be
competition or resources and not
every individual will obtain enough
to allow them to survive and
reproduce.
Figure 4 The breeding rate of pairs of
southern ground hornbills, Bucorvus
leadbeateri, is as low as 0.3 young per year
251
5 Evolution and biodivErsity
Activity diferential survival an reprouction
simulation of natural Individuals that are better adapted tend to survive and
election produce more ospring while the less well adapted tend
to die or produce ewer ospring.
Make ten or more
artifcial fsh using Chance plays a part in deciding which individuals survive and reproduce
modelling clay, or some and which do not, but the characteristics o an individual also have an
other malleable material. infuence. In the struggle or existence the less well-adapted individuals
Drop each o them into tend to die or ail to reproduce and the best adapted tend to survive and
a measuring cylinder o produce many ospring. This is natural selection.
water and time how long
each takes to reach the An example that is oten quoted is that o the girae. It can graze on
bottom. grass and herbs but is more adapted to browse on tree leaves. In the wet
season its ood is abundant but in the dry season there can be periods
Discard the hal o o ood shortage when the only remaining tree leaves are on high
the models that were branches. Giraes with longer necks are better adapted to reaching
slowest. Pair up the these leaves and surviving periods o ood shortage than those with
astest models and shorter necks.
make intermediate
shapes, to represent Inheritance
their ospring. Random
new shapes can also be Individuals that reproduce pass on characteristics
introduced to simulate to their ospring.
mutation.
Much o the variation between individuals can be passed on to
Test the new generation ospring it is heritable. Maasai children inherit the dark skin colour
and repeat the o their parents or example and children o light-skinned north
elimination o the European parents inherit a light skin colour. Variation in behaviour can
slowest and the breeding be heritable. The direction o migration to overwintering sites in the
o the astest. Does blackcap Sylvia atricapilla is an example. D ue to dierences in their genes,
one shape gradually some birds o this species migrate southwestwards rom Germany to
emerge? Describe its Spain or the winter and others northwestwards to Britain.
eatures.
Not all eatures are passed on to ospring. Those acquired during the
lietime o an individual are not usually inherited. An elephant with a
broken tusk does not have calves with broken tusks or example. I a
person develops darker skin colour through exposure to sunlight, the
darker skin is not inherited. Acquired characteristics are thereore not
signicant in the evolution o a species.
Progressive change
Natural selection increases the requency o
characteristics that make individuals better adapted and
decreases the requency o other characteristics leading
to changes within the species.
B ecause better-adapted individuals survive, they can reproduce and
pass on characteristics to their ospring. Individuals that are less well
adapted have lower survival rates and less reproductive success. This
leads to an increase in the proportion o individuals in a population with
252
5.2 n AturAl sElEction
characteristics that make them well adapted. Over the generations, the Avy
characteristics o the population gradually change this is evolution by
natural selection. The impulse to reproduce and pass
on characteristics can be very strong.
Major evolutionary changes are likely to occur over long time periods It can cause adult males to carry out
and many generations, so we should not expect to be able to observe infanticide. How could this behaviour
them during our lietime, but there are many examples o smaller but pattern have evolved in lions and
signicant changes that have been observed. The evolution o dark wing other species? Female cheetahs mate
colours in moths has been observed in industrial areas with polluted with two or more males so their litters
air. Two examples o evolution are described in the next sections o have multiple paternity. How does this
this book: changes to beaks o nches on the Galapagos Islands and the protect the young against infanticide?
development o antibiotic resistance in bacteria.
Figure 5 A female cheetahs cubs inherit
Daa-baed qe: Evolution in rice plants characteristics from her and from one of
the several males with whom she mated
The bar charts in gure 6 show the results o an investigation o
evolution in rice plants. F1 hybrid plants were bred by crossing together 253
two rice varieties. These hybrids were then grown at ve dierent sites
in Japan. Each year the date o fowering was recorded and seed was
collected rom the plants, or re-sowing at that site in the ollowing year.
F F F F
3 4 5
Sapporo
43 N
Fujisaka
40 N
Konasu
36 N
single H i ra tsu ka
original 35 N
population
planted Chikugo
out at 33 N
Miyazaki
31 N
56 70 84 98 112 126 68 82 96 110 124 138 54 68 82 96 110124138 51 65 79 93 107121 135
days to owering
Figure 6
1 Why was the investigation done using hybrids rather than a
single pure-bred variety? [2]
2 Describe the changes, shown in the chart, between the F3 and
F6 generations o rice plants grown at Miyazaki. [2]
3 a) State the relationship between fowering time and latitude
in the F6 generation. [1 ]
b) Suggest a reason or this relationship. [1 ]
4 a) Predict the results i the investigation had been carried on
until the F generation. [1 ]
10
b) Predict the results o collecting seeds rom F10 plants grown at
Sapporo and rom F10 plants grown at Miyazaki and sowing
them together at Hiratsuka. [3]
5 Evolution and biodivErsity
Galpagos fnches
Changes in beaks o fnches on Daphne Major.
Pinta (5) Genovesa (4)
Rabida (8) Marchena (4)
Santiago (10)
Daphne Major (2/3)
Fernandina Santa Cruz San Cristbal (a) G. fortis (large beak)
(9) (9) (7)
Isabela (10) Santa Fe
(5)
Santa Maria (8) Espaola (3)
Figure 7 The Galpagos archipelago with the number
o species o fnch ound on each island
Darwin visited the Galpagos Islands in 1 835 (b) G. fortis (small beak)
and collected specimens o small birds, which
were subsequently identifed as fnches. There are (c) G. magnirostris
1 4 species in all. Darwin observed that the sizes and
shapes o the beaks o the fnches varied, as did their Figure 8 Variation in beak shape in Galpagos fnches.
diet. From the overall similarities between the birds (a) G. fortis (large beak) . (b) G. fortis (small beak) .
and their distribution over the Galapagos islands (c) G. magnirostris
(see fgure 7) , Darwin hypothesized that one might
really ancy that rom an original paucity o birds among individuals with shorter beaks. In 1 98283
in this archipelago, one species had been taken and there was a severe El Nio event, causing eight
modifed or dierent ends. months o heavy rain and as a result an increased
supply o small, sot seeds and ewer large, hard
There has since been intense research into seeds. G. fortis bred rapidly, in response to the
what have become known as Darwins fnches. increase in ood availability. With a return to dry
In particular, Peter and Rosemary Grant have weather conditions and greatly reduced supplies
shown that beak characters and diet are closely o small seeds, breeding stopped until 1 987. In
related and when one changes, the other does that year, only 3 7 per cent o those alive in 1 983
also. A particular ocus o Peter and Rosemary bred and they were not a random sample o the
Grants research has been a population o the 1 983 population. In 1 987, G. fortis had longer and
medium ground fnch, Geospiza fortis, on a small narrower beaks than the 1 983 averages, correlating
island called D aphne Maj or. O n this island, the with the reduction in supply o small seeds.
small ground fnch, Geospiza fuliginosa, is almost Variation in the shape and size o the beaks ( see
absent. Both species eed on small seeds, though fgure 8) is mostly due to genes, though the
G. fortis can also eat larger seeds. In the absence
o competition rom G. fuliginosa or small seeds,
G. fortis is smaller in body size and beak size on
Daphne Major than on other islands.
In 1 977, a drought on Daphne Major caused a
shortage o small seeds, so G. fortis ed instead
on larger, harder seeds, which the larger-beaked
individuals are able to crack open. Most o the
population died in that year, with highest mortality
254
5.2 n AturAl sElEction
environment has some eect. The proportion o One o the objections to the theory o evolution
the variation due to genes is called heritability. by natural selection is that signifcant changes
Using the heritability o beak length and width caused by natural selection have not been
and data about the birds that had survived to observed actually occurring. It is unreasonable to
breed, the changes in mean beak length and expect huge changes to have occurred in a species,
width between 1 983 and 1 987 were predicted. even i it had been ollowed since Darwins theory
The observed results are very close to the was published in 1 85 9, but in the case o G. fortis,
predictions. Average beak length was predicted to signifcant changes have occurred that are clearly
increase by 1 0 m and actually increased by 6 m. linked to natural selection.
Average beak width was predicted to decrease by
1 30 m and actually decreased by 1 20 m.
Daa-baed qe: Galpagos fnches the changes in the population o
When Peter and Rosemary Grant began to study G. magnirostris. [3]
fnches on the island o Daphne Major in 1 973,
there were breeding populations o two species, 2 Daphne Major has an area o 0.34 km2.
Geospiza fortis and Geospiza scandens. Geospiza 1 km2 is 1 00 hectares and 1 hectare is 1 00
magnirostris established a breeding population on
the island in 1 982, initially with just two emales 1 00 m. Calculate the maximum and
and three males. Figure 9 shows the numbers
o G. magnirostris and G. fortis on D aphne Maj or minimum population densities o G. ortis
between 1 997 and 2006.
during 1 9972006. [4]
1500 G. fortis
G. magnirostrisnumbersTable 2 shows the percentages o three types o
seed in the diets o the three fnch species on
1000 D aphne Maj or. Small seeds are produced by 2 2
plant species, medium seeds by the cactus Opuntia
echios, and large seeds, which are very hard, by
Tribulus cistoides.
500 3 a) Outline the diet o each o the species
o fnch on D aphne Maj or. [3]
0 b) There was a very severe drought on
1996 1998 2000 2002 2004 2006 Daphne Major in 2003 and 2004.
Deduce how the diet o the fnches
year changed during the drought, using
the data in the table.
Figure 9 Changes in numbers of G. fortis and G. magnirostris
between 1996 and 2006
1 a) Describe the changes in the population [3]
o G. magnirostris between 1 997 4 Figure 1 0 shows an index o beak size o adult
G. fortis rom 1 973 to 2 006, with the size in
and 2006. [2] 1 973 assigned the value zero and the sizes in
other years shown in comparison to this.
b) Compare the changes in population o
G. fortis between 1 997 and 2 006 with
spee 1977 Geospiza fortis 2004 Geospiza magnirostris Geospiza scandens
Yea 75 1985 1989 80 1985 1989 2004 1977 1985 1989 2004
sma 10 11
17 80 77 8.2 18 5.9 4.5 85 77 23 17
Medm 0.0 5.1 0.0 12 26 15 22 70 83
lage 19 16 82 82 69 0.0 0.0 0.0 0.0
Table 2
255
5 Evolution and biodivErsity
1 c) In the frst severe drought, the mean
beak size o G. fortis increased, but in the
0.5 second drought, it decreased. Using the
data in this question, explain how natural
beak size index 0 selection could cause these changes in
beak size in the two droughts. [3]
-0.5 5 The intensity o natural selection on Daphne
Major was calculated during the two
-1 droughts. The calculated values are called
selection dierentials. They range rom 1 .08
-1.5 or beak length during the second drought,
1975 1980 1985 1990 1995 2000
year 2005 to +0.88 or beak length in the frst drought,
Figure 10 Relative beak size in G. fortis between with similar selection dierentials or beak
1973 and 2006
width and depth and overall beak size.
These are very large selection dierentials,
compared to values calculated in other
The graph shows two periods o very rapid investigations o evolution.
change in mean beak size, both o which
correspond with droughts on D aphne Maj or. Suggest reasons or natural selection on the
beak size o G. fortis being unusually intense
a) State two periods o most rapid change on the island o D aphne Maj or. [2]
in mean beak size o G. fortis. [2] 6 Discuss the advantages o investigations
b) Suggest two reasons or mean beak size o evolution over long periods and the reasons
changing most rapidly when there is or ew long-term investigations
a drought. [2] being done. [3]
natural selectio ad atibiotic resistace
Use theories to explain natural phenomena: the theory of evolution by natural
selection can explain the development of antibiotic resistance in bacteria.
Antibiotics were one o the great triumphs o development o antibiotic resistance is thereore
medicine in the 2 0th century. When they were an example o evolution. It can be explained in
frst introduced, it was expected that they would terms o the theory o natural selection. A scientifc
oer a permanent method o controlling bacterial understanding o how antibiotic resistance
diseases, but there have been increasing problems develops is very useul as it gives an understanding
o antibiotic resistance in pathogenic bacteria. o what should be done to reduce the problem.
The ollowing trends have become established: 16% resistant
14
Ater an antibiotic is introduced and used on 12
patients, bacteria showing resistance appear 10
within a ew years.
8
Resistance to the antibiotic spreads to more 6
and more species o pathogenic bacteria. 4
2
In each species the proportion o inections 0
that are caused by a resistant strain increases.
Figure 11 Percentage resistance to ciprofoxacin between
So, during the time over which antibiotics 1990 and 2004
have been used to treat bacterial diseases there 1990
have been cumulative changes in the antibiotic 1991
resistance properties o populations o bacteria. The 1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
256
5.2 n AturAl sElEction
antibiotic resistnce
Evolution of antibiotic resistance in bacteria.
Antibiotic resistance is due to genes in bacteria and population with no
so it can be inherited. The mechanism that causes antibiotic-resistant bacteria
antibiotic resistance to become more prevalent or
to diminish is summarized in gure 1 2. antibiotic resistance antibiotic resistance
gene received from a gene formed by
The evolution o multiple antibiotic resistance bacterium in another mutation in one
has occurred in just a ew decades. This rapid bacterium
evolution is due to the ollowing causes: population
There has been very widespread use o population with some
antibiotics, both or treating diseases and in antibiotic-resistant bacteria
animal eeds used on arms.
antibiotic is used therefore
B acteria can reproduce very rapidly, with a there is strong natural
generation time o less than an hour. selection for resistance
Populations o bacteria are oten huge, population with more
increasing the chance o a gene or antibiotic antibiotic-resistant bacteria
resistance being ormed by mutation.
antibiotic is not used therefore
Bacteria can pass genes on to other bacteria in there is natural selection
several ways, including using plasmids, which (weak) against resistance
allow one species o bacteria to gain antibiotic
resistance genes rom another species. population with slightly fewer
antibiotic-resistant bacteria
Figure 12 Evolution o antibiotic resistance
Daa-baed qe: Chlortetracycline resistance in soil bacteria
Bacteria were collected rom soil at dierent desistance (%) 3.0
distances rom a site on a pig arm in Minnesota
where manure had been allowed to overfow 2.5
rom an animal pen and accumulate. The
eed given to the pigs on this arm contained 2.0
subtherapeutic low doses o the antibiotic
chlortetracycline, in order to promote aster 1.5
growth rates. The bacteria were tested to nd
out what percentage o them was resistant to 1.0
this antibiotic. The results are shown in the bar
chart. The yellow bars show the percentage o 0.5
chlortetracycline resistant bacteria that grew on
nutrient-rich medium and the orange bars show 0.0
the percentage on a nutrient-poor medium that 5 m 20 m 100 m
encouraged dierent types o bacteria to grow.
distance from animal pen
Source: " The efects o subtherapeutic antibiotic use in arm animals
on the prolieration and persistence o antibiotic resistance among soil
bacteria", Sudeshna Ghosh and Timothy M LaPara, The International
Society for Microbial Ecology Journal (2007) 1, 191203
1 a) State the relationship between percentage
antibiotic resistance and distance rom the 2 Predict whether the percentage antibiotic
[1 ] resistance would have been lower at 200 metres
animal pen. rom the pen than at 1 00 metres. [3]
b) Explain the dierence in antibiotic
resistance between populations o bacteria 3 Discuss the use o subtherapeutic doses o
[4] antibiotics in animal eeds. [2]
near and ar rom the pen.
257
5 Evolution and biodivErsity
5.3 classifation o biodiversity
udertdig applictio
The binomial system o names or species is Classifcation o one plant and one animal
universal among biologists and has been agreed species rom domain to species level.
and developed at a series o congresses.
External recognition eatures o bryophytes,
When species are discovered they are given flicinophytes, conierophytes and
scientifc names using the binomial system. angiospermophytes.
Taxonomists classiy species using a hierarchy Recognition eatures o poriera, cnidaria,
o taxa. platyhelminthes, annelida, mollusca and
arthropoda, chordata.
All organisms are classifed into three domains.
Recognition o eatures o birds, mammals,
The principal taxa or classiying eukaryotes are amphibians, reptiles and fsh.
kingdom, phylum, class, order, amily, genus
and species. skill
In a natural classifcation the genus and Construction o dichotomous keys or use in
accompanying higher taxa consist o all the identiying specimens.
species that have evolved rom one common
ancestral species. ntre o ciece
Taxonomists sometimes reclassiy groups Cooperation and collaboration between groups
o species when new evidence shows that a o scientists: scientists use the binomial
previous taxon contains species that have system to identiy a species rather than the
evolved rom dierent ancestral species. many dierent local names.
Natural classifcations help in identifcation
o species and allow the prediction o
characteristics shared by species within
a group.
Itertiol coopertio d clifctio
Cooperation and collaboration between groups o scientists: scientists use the
binomial system to identiy a species rather than the many dierent local names.
Recognizable groups of organisms are known to la chandelle, le pied-de-veau, le manteau de
biologists as species. The same species can have la Sainte-Vierge, la pilette or la vachotte. In
many different local names, even within one Spanish there are even more names for this one
language. For example, in England the species species of which these are just a few: comida
of plant known to scientists as Arum maculatum de culebra, alcatrax, barba de arn, dragontia
has been called lords-and-ladies, cuckoo- menor, hoj as de fuego, vela del diablo and yerba
pint, jack in the pulpit, devils and angels, del quemado. The name primaveras is used for
cows and bulls, willy lily and snakes meat. In Arum maculatum in S panish but for a different
French there is also a variety of local names: plant in other languages.
258
5.3 clAssificAtion of BioDi vErsitY
Local names may be a valuable part o the Seeblumen and geel Seeblumen (used by Fuchs) ,
culture o an area, but science is an international English wild mynte and water mynte (used by
venture so scientifc names are needed that are Turner) and Malayan jambu bol and jambu chilli
understood throughout the world. The binomial (applied by Malays to dierent species o Eugenia) .
system that has developed is a good example o
cooperation and collaboration between scientists. Figure 1 Arum maculatum
The credit or devising our modern system o
naming species is given to the Swedish biologist
Carl Linnaeus who introduced a system o two-
part names in the 1 8th century. This stroke o
genius was the basis or the binomial system that
is still in use today. In act Linnaeus was mirroring
a style o nomenclature that had been used in
many languages beore. The style recognizes that
there are groups o similar species, so the name or
each species in a group consists o a specifc name
attached to the group name, as in the Ancient
Greek and
(used by Threophrastus) , Latin anagallis mas and
anagallis femina (used by Pliny) , German weiss
development of the binomial system Figure 2 Linnaea borealis. Binomials
are often chosen to honour a biologist,
The binomial system of names for species is universal or to describe a feature of the
among biologists and has been agreed and developed organism. Linnaea borealis is named
at a series of congresses. in honour of Carl Linnaeus, the Swedish
biologist who introduced the binomial
To ensure that all biologists use the same system o names or living system of nomenclature and named
organisms, congresses attended by delegates rom around the world are many plants and animals using it
held at regular intervals. There are separate congresses or animals and
or plants and ungi. 259
International Botanical Congresses (IBC) were held every year during
the late 1 9th century. The IB C held in Genoa in 1 892 proposed that
1 753 be taken as the starting point or both genera and species o
plants and ungi as this was the year when Linnaeus published Species
Plantarum, the book that gave consistent binomials or all species o the
plant kingdom then known. The IB C o Vienna in 1 905 accepted by
1 50 votes to 1 9 the rule that La nomenclature botanique commence
avec Linn, Species Plantarum ( ann. 1 75 3 ) pour les groupes de plantes
vasculaires. The 1 9th IBC will be in Shenzhen, China, in 201 7.
The frst International Zoological Congress was held in Paris in 1 889.
It was recognized that internationally accepted rules or naming and
classiying animal species were needed and these were agreed at this
and subsequent congresses. 1 758 was chosen as the starting date or
valid names o animal species as this was when Linnaeus published
Systema Natura in which he gave binomials or all species known then.
The current International Code or Zoological Nomenclature is the
4th edition and there will no doubt be more editions in the uture as
scientists refne the methods that they use or naming species.
5 Evolution and biodivErsity
ALLI G ATO R I D AE m is s iss ip p ie n s is the binomial sysem
Alligator sinensis
When species are discovered they are given scientifc
Ca im a n crocodilus names using the binomial system.
latirostris
yacare The system that biologists use is called binomial nomenclature, because
the international name o a species consists o two words. An example is
Melano- niger Linnaea borealis ( fgure 2 ) . The frst name is the genus name. A genus is
suchus a group o species that share certain characteristics. The second name
is the species or specifc name. There are various rules about binomial
Paleo- palpebrosus nomenclature:
suchus trig o n a tu s
The genus name begins with an upper-case ( capital) letter and the
Figure 3 Classifcation o the alligator amily species name with a lower-case ( small) letter.
In typed or printed text, a binomial is shown in italics.
Ater a binomial has been used once in a piece o text, it can be
abbreviated to the initial letter o the genus name with the ull
species name, or example: L. borealis.
The earliest published name or a species, rom 1 753 onwards or
plants or 1 758 or animals, is the correct one.
the hierarchy of axa
Taxonomists classiy species using a hierarchy o taxa.
The word taxon is Greek and means a group o something. The plural is
taxa. In biology, species are arranged or classifed into taxa. Every species
is classifed into a genus. Genera are grouped into amilies. An example
o the genera and species in a amily is shown in fgure 3. Families are
grouped into orders, orders into classes and so on up to the level o
kingdom or domain. The taxa orm a hierarchy, as each taxon includes
taxa rom the level below. Going up the hierarchy, the taxa include larger
and larger numbers o species, which share ewer and ewer eatures.
the hree domains
All organisms are classifed into three domains.
Traditional classifcation systems have recognized two maj or categories
o organisms based on cell types: eukaryotes and prokaryotes. This
classifcation is now regarded as inappropriate because the prokaryotes
have been ound to be very diverse. In particular, when the base
sequence o ribosomal RNA was determined, it became apparent that
there are two distinct groups o prokaryotes. They were given the names
Eubacteria and Archaea.
Most classifcation systems thereore now recognize three major categories
o organism, Eubacteria, Archaea and Eukaryota. These categories are
called domains, so all organisms are classifed into three domains. Table 1
shows some o the eatures that can be used to distinguish between them.
Members o the domains are usually reerred to as bacteria, archaeans
and eukaryotes. Bacteria and eukaryotes are relatively amiliar to most
biologists but archaeans are oten less well known.
260
5.3 clAssificAtion of BioDi vErsitY
feaue Dma
Baea Ahaea Eukaya
Histones associated Absent Proteins similar to histones Present
with DNA bound to DNA
Presence o introns Rare or absent Present in some genes Frequent
Structure o cell walls Made o chemical called Not made o peptidoglycan Not made o peptidoglycan;
peptidoglycan not always present
Cell membrane Glycerol-ester lipids; Glycerol-ether lipids; Glycerol-ester lipids;
dierences unbranched side chains;
d-orm o glycerol unbranched side chains; l-orm unbranched side chains;
o glycerol d-orm o glycerol
Table 1
Archaeans are ound in a broad range o habitats such as the ocean surace, Ay
deep ocean sediments and even oil deposits ar below the surace o the
Earth. They are also ound in some airly extreme habitats such as water ideyg a kgdm
with very high salt concentrations or temperatures close to boiling. The This is a defnition o the
methanogens are obligate anaerobes and give o methane as a waste product characteristics o organisms in
o their metabolism. Methanogens live in the intestines o cattle and the guts one o the kingdoms. Can you
o termites and are responsible or the production o marsh gas in marshes. deduce which kingdom it is?
Viruses are not classifed in any o the three domains. Although they Multicellular; cells typically
have genes coding or proteins using the same genetic code as living held together by intercellular
organisms they have too ew o the characteristics o lie to be regarded junctions; extracellular
as living organisms. matrix with brous proteins,
typically collagens, between
Bacteria Archaea Eukaryota two dissimilar epithelia;
sexual with production ofan
Green lamentous Slime egg cell that is fertilized by a
smaller, often monociliated,
Spirochetes bacteria molds Animals sperm cell; phagotrophic and
Proteobacteria Gram Methanobacterium Halophiles Fungi osmotrophic; without cell wall.
positives Methanococcus
Plants Figure 5 Brown seaweeds have
been classifed in the kingdom
Cy an o b a cte ria Ciliates Protoctista
Flagellates 261
Figure 4 Tree diagram showing relationships between living organisms based on base
sequences o ribosomal RNA
Eukaryote classifcation
The principal taxa or classiying eukaryotes are kingdom,
phylum, class, order, amily, genus and species.
Eukaryotes are classifed into kingdoms. Each kingdom is divided up
into phyla, which are divided into classes, then orders, amilies and
genera. The hierarchy o taxa or classiying eukaryotes is thus kingdom,
phylum, class, order, amily, genus and species.
Most biologists recognize our kingdoms o eukaryote: plants, animals,
ungi and protoctista. The last o these is the most controversial
as protoctists are very diverse and should be divided up into more
kingdoms. At present there is no consensus on how this should be done.
5 Evolution and biodivErsity
Examples o classifcatio
Classifcation o one plant and one animal species rom
domain to species level.
Animals and plants are kingdoms o the domain Eukaryota. Table 2
shows the classication o one plant and one animal species rom
kingdom down to species.
taxon Grey wolf Dae palm
Kingdom Animalia Plantae
Phylum Chordata Angiospermophyta
Class Mammalia Monocotyledoneae
Order Carnivora Palmales
Family Canidae Arecaceae
Genus Canis Phoenix
Species lupus dactylifera
Table 2
Daa-based quesions: Classiying cartilaginous fsh
All the sh shown in gure 6 are in the class 1 State the kingdom to which all o the species
Chondrichthyes. They are the most requently
ound sh in this class in north-west Europe. in gure 6 belong. [1 ]
2 a) Four o the sh in gure 6 are classied in
the same genus. Deduce which these sh
are. [1 ]
b) Deduce with a reason whether these our
sh are in:
(i) the same or dierent species [2]
(ii) the same or dierent amilies. [2]
c) State two characteristics o these our
sh that are not possessed by the other
our sh. [2]
3 The other our sh are classied into two
orders. Deduce, with a reason, how the our
Figure 6 Cartilaginous fsh in seas in north-west Europe sh are split into two orders. [2]
natural classifcatio
In a natural classifcation, the genus and accompanying higher taxa consist o all the
species that have evolved rom one common ancestral species.
Scientic consensus is to classiy species in a way An example o an unnatural or articial
that most closely ollows the way in which species classication would be one in which birds, bats
evolved. Following this convention, all members and insects are grouped together, because they
o a genus or higher taxon should have a common all fy. Flight evolved separately in these groups
ancestor. This is called a natural classication. Because and as they do not share a common ancestor they
o the common ancestry we can expect the members dier in many ways. It would not be appropriate
o a natural group to share many characteristics. to classiy them together other than to place them
262
5.3 clAssiicAtion o BioDi vErsitY
all in the animal kingdom and both birds and bats distantly related organisms appear superfcially
in the phylum Chordata. Plants and ungi were at similar and adaptive radiation can make closely
one time classifed together, presumably because related organisms appear dierent. In the past,
they have cell walls and do not move, but this is natural classifcation was attempted by looking at
an artifcial classifcation as their cell walls evolved as many visible characteristics as possible, but new
separately and molecular research shows that they molecular methods have been introduced and these
are no more similar to each other than to animals. have caused signifcant changes to the classifcation
o some groups. More details o this are given later,
It is not always clear which groups o species do in sub-topic 5.4.
share a common ancestor, so natural classifcation
can be problematic. Convergent evolution can make
TOK
Wha a fuee he deepme a e eu?
Carl Linnaeuss 1753 book Species Plantarum introduced genera and species. This was incorporated in the American
consistent two-part names (binomials) or all species o Rochester Code o1883 and in the code used at the Berlin
the vegetable kingdom then known. Thus the binomial Botaniches Museum and supported by British Museum o
Physalis angulata replaced the obsolete phrase-name, Natural History, Harvard University botanists and a group
oSwiss and Belgian botanists. The International Botanical
Physalis annua ramosissima, ramis angulosis glabris, Congress oVienna in 1905 accepted by 150 votes to 19
foliis dentato-serratis. Linnaeus brought the scientifc the rule that La nomenclature botanique commence avec
nomenclature oplants back to the simplicity and brevity Linn, Species Plantarum (ann. 1753) pour les groupes de
othe vernacular nomenclature out owhich it had grown. plantes vasculaires.
Folk-names or species rarely exceed three words. In
groups ospecies alike enough to have a vernacular 1 Why was Linnaeuss system or naming plants adopted
group-name, the species are oten distinguished by a as the international system, rather than any other
single name attached to the group-name, as in the Ancient system?
Greek and
(used by Threophrastus), Latin anagallis mas and anagallis 2 Why do the international rules onomenclature state
emina (used by Pliny), German weiss Seeblumen and geel that genus and species names must be in Ancient
Seeblumen (used by Fuchs), English wild mynte and water Greek or Latin?
mynte (used by Turner) and Malayan jambu bol and jambu
chilli (applied by Malays to dierent species oEugenia). 3 Making decisions by voting is rather unusual in science.
Why is it done at International Botanical Congresses?
The International Botanical Congress held in Genoa in 1892 What knowledge issues are associated with this
proposed that 1753 be taken as the starting point or both method odecision making?
reviewing classifcation
Taxonomists sometimes reclassiy groups o species
when new evidence shows that a previous taxon contains
species that have evolved rom dierent ancestral species.
Sometimes new evidence shows that members o a group do not share a
common ancestor, so the group should be split up into two or more taxa.
Conversely species classifed in dierent taxa are sometimes ound to
be closely related, so two or more taxa are united, or species are moved
rom one genus to another or between higher taxa.
The classifcation o humans has caused more controversy than any
other species. Using standard taxonomic procedures, humans are
assigned to the order Primates and the amily Hominidae. There has
been much debate about which, i any, o the great apes to include in
this amily. O riginally all the great apes were placed in another amily,
263
5 Evolution and biodivErsity
the Pongidae, but research has shown that chimpanzees and gorillas
are closer to humans than to orang-utans and so should be in the
same amily. This would j ust leave orang- utans in the Pongidae. Most
evidence suggests that chimpanzees are closer than gorillas to humans,
so i humans and chimpanzees are placed in dierent genera, gorillas
should also be in a separate genus. A summary o this scheme or human
classication is shown in gure 7.
FAM I LY Hominidae Pongidae
GENUS AND Gorilla Homo Pan Pan Po n g o
SPECIES gorilla sapiens troglodytes paniscus pygmaeus
( go ri l l a ) (human) (chimpanzee) (bonobo) ( o ra n g-u t a n )
Figure 7 Classifcation o humans
Figure 8 Members o the Hominidae advntges o nturl clssifction
and Pongidae
Natural classications help in identication o species
Ativity and allow the prediction o characteristics shared by
species within a group.
controlling potato blight
Phytophthora infestans, the There is great interest at the moment in the biodiversity o the world. Groups
organism that causes the disease o biologists are surveying areas where little research has been done beore,
potato blight, has hyphae and to nd out what species are present. Even in well-known parts o the world
was classied as a ungus, but new species are sometimes discovered. Natural classication o species is very
molecular biology has shown that it helpul in research into biodiversity. It has two specic advantages.
is not a true ungus and should be
classied in a dierent kingdom, 1 Identication o species is easier. I a specimen o an organism is
possibly the Protoctista. Potato ound and it is not obvious what species it is, the specimen can be
blight has proved to be a difcult identied by assigning it rst to its kingdom, then the phylum within
disease to control using ungicides. the kingdom, class within the phylum and so on down to species
Discuss reasons or this. level. Dichotomous keys can be used to help with this process. This
process would not work so well with an articial classication. For
264 example, i fowering plants were classied according to fower
colour and a white- fowered bluebell Hyacinthoides non-scripta
was discovered, it would not be identied correctly as the species
normally has blue fowers.
2 Because all o the members o a group in a natural classication
have evolved rom a common ancestral species, they inherit similar
characteristics. This allows prediction o the characteristics o species
within a group. For example, i a chemical that is useul as a drug
is ound in one plant in a genus, this or related chemicals are likely
to be ound in other species in the genus. I a new species o bat
was discovered, we could make many predictions about it with
reasonable certainty that they are correct: the bat will have hair,
mammary glands, a placenta, a our-chambered heart and many
other mammalian eatures. None o these predictions could be made
i bats were classied articially with all other fying organisms.
5.3 clAssificAtion of BioDi vErsitY
dichotomous keys
Construction o dichotomous keys or use in identiying specimens
Dichotomous keys are oten constructed to use or 1 Fore and hind limbs visible, can emerge on land ..... 2
identiying species within a group. A dichotomy Only ore limbs visible, cannot live on land ................ 6
is a division into two; a dichotomous key consists
o a numbered series o pairs o descriptions. One 2 Fore and hind limbs have paws ..................................... 3
o these should clearly match the species and Fore and hind limbs have fippers ................................. 4
the other should clearly be wrong. The eatures
that the designer o the key chooses to use in the 3 Fur is dark ............................................................ sea otters
descriptions should thereore be reliable and easily Fur is white ........................................................ polar bears
visible. Each o the pair o descriptions leads either
to another o the numbered pairs o descriptions 4 External ear fap visible ........... sea lions and ur seals
in the key, or to an identifcation. No external ear fap ........................................................... 5
An example o a key is shown in table 3 . We can 5 Two long tusks ..................................................... walruses
use it to identiy the species in fgure 9. In the frst No tusks ............................................................... true seals
stage o the key, we must decide i hind limbs are
visible. They are not, so we are directed to stage 6 Mouth breathing, no blowhole ... dugongs and manatees
6 o the key. We must now decide i the species Breathing through blowholes ......................................... 7
has a blowhole. It does not, so it is a dugong or a
manatee. A uller key would have another stage 7 Two blowholes, no teeth ......................... baleen whales
to separate dugongs and manatees. One blowhole, teeth ........ dolphins, porpoises and whales
Table 3 Key to groups of marine mammals
Ay Figure 9 Manatee
cug dhmu key
Keys are usually designed or use in a particular area. All the groups or species
that are ound in that area can be identied using the key. There may be a
group o organisms in your area or which a key has never been designed.
You could design a key to the trees in the local orest or on your school
campus, using lea descriptions or bark descriptions.
You could design a key to birds that visit bird-eeding stations in your area.
You could design a key to the invertebrates that are associated with one
particular plant species.
You could design a key to the ootprints o mammals and birds (gure 10) .
They are all right ront ootprints and are not shown to scale.
bear wolf fox cat dog
duck rabbit / hare squirrel deer heron
Figure 10 Footprints of mammals and birds
265
5 Evolution and biodivErsity
Plants
External recognition eatures o bryophytes, licinophytes, conierophytes
and angiospermophytes.
All plants are classied together in one kingdom. example is in one o the smaller phyla. The our
In the lie cycle o every plant, male and emale main plant phyla are:
gametes are ormed and use together. The zygote
ormed develops into an embryo. The way in Bryophyta mosses, liverworts and hornworts
which this embryo develops depends on the type
o plant it is. The dierent types o plants are put Filicinophyta erns
into phyla.
Conierophyta coniers
Most plants are in one o our phyla, but there
are other smaller phyla. The Ginkgo biloba tree or Angiospermophyta fowering plants.
The external recognition eatures o these phyla
are shown in table 4.
Bryophyta filiinophyta conierophyta Angiospermophyta
Vegetative organs parts Rhizoids but no Roots, stems and leaves are usually present
o the plant concerned true roots. Some
with growth rather than with simple stems
reproduction and leaves; others
have only a thallus
Vascular tissue tissues No xylem or Xylem and phloem are both present
with tubular structures used phloem
or transport within the plant
Cambium cells between No cambium; no true trees and Present in coniers and most angiosperms,
xylem and phloem that shrubs allowing secondary thickening ostems and
can produce more o these roots and development oplants into trees
tissues and shrubs
Pollen small structures Pollen is not produced Pollen is produced Pollen is produced
containing male gametes in male cones by anthers in
that are dispersed fowers
Ovules contains a emale No ovaries or ovules Ovules are produced Ovules are enclosed
gamete and develops into a in emale cones inside ovaries in
seed ater ertilization
fowers
Seeds dispersible unit No seeds Seeds are produced and dispersed
consisting o an embryo
plant and ood reserves,
inside a seed coat
Fruits seeds together with No ruits Fruits produced or
a ruit wall developed rom dispersal o seeds
the ovary wall by mechanical, wind
or animal methods
Table 4
266
5.3 clAssificAtion of BioDi vErsitY
animl phyl
Recognition eatures o poriera, cnidaria, platyhelminthes, annelida, mollusca and
arthropoda, chordata.
Animals are divided up into over 30 phyla, based on their characteristics. Six phyla are eatured in
table 5 . Two examples o each are shown in fgure 1 1 .
Phyum Muh/au symmey skee ohe exea
No mouth or None eg eaue
Poriera an sponges, anus Internal spicules
cup sponges, tube (sketetal needles) Many pores over the surace
sponges, glass sponges Mouth only Radial through which water is drawn
Sot, but hard in or lter eeding. Very varied
Cnidaria hydras, Mouth only Bilateral corals secrete shapes
jellysh, corals, sea CaCO3
anemones Mouth and Bilateral Tentacles arranged in rings
anus Sot, with no around the mouth, with stinging
Platyhelminthes skeleton cells. Polyps or medusae
fatworms, fukes, Mouth and Bilateral (jellysh)
tapeworms anus Most have shell
Mollusca bivalves, made o CaCO3 Flat and thin bodies in the shape
gastropods, snails, Mouth and Bilateral o a ribbon. No blood system or
chitons, squid, octopus anus Internal cavity system or gas exchange
with fuid under
Annelida marine pressure A old in the body wall called
bristleworms, the mantle secretes the shell. A
oligochaetes, leeches External skeleton hard rasping radula is used or
made o plates o eeding
Arthropoda insects, chitin
arachnids, crustaceans, Bodies made up o many ring-
myriapods shaped segments, oten with
bristles. Blood vessels oten
visible
Segmented bodies and legs or
other appendages with joints
between the sections
Table 5 Characteristics of six animal phyla 3 List the organisms that have: [3]
[7] a) jointed appendages [2]
1 Study the organisms shown in fgure 1 1
and assign each one to its phylum. b) stinging tentacles
2 List the organisms that are: c) bristles.
a) bilaterally symmetric
b) radially symmetric 4 List the organisms that flter eed by
c) not symmetrical in their structure. [3] pumping water through tubes inside
their bodies.
267
5 Evolution and biodivErsity
Adocia cinerea Alcyonium glomeratum vertebrates
Nymphon gracilis Pycnogonum littorale Recognition o eatures o birds, mammals, amphibians,
reptiles and fsh.
Corynactis viridis Lepidonotus clara
Most species o chordate belong to one o fve major classes, each o
Polymastia mammiliaris Cyanea capillata which contains more than a thousand species. Although the numbers
Procerodes littoralis are not certain and new species are still sometimes discovered, there
Loligo forbesii are about 1 0,000 bird species, 9,000 reptiles, 6,000 amphibians and
Arenicola marina 5,700 mammals. All o these classes are outnumbered by the ray-fnned
Prostheceraeus vittatus bony fsh, with more than 30,000 species. The recognition eatures o the
Caprella linearis fve largest classes o chordate are shown in table 6. All o the organisms
Gammarus locusta are vertebrates, because they have a backbone composed o vertebrae.
Figure 11 Invertebrate diversity Bony ay- Amphibians reptiles Bids Mammals
fnned fsh
268 Sot moist I m p e rm e a b l e Skin with Skin has
Scales which skin skin covered
are bony permeable in scales o eathers made ollicles with
plates in the to water and keratin
skin gases o keratin hair made o
keratin
Gills covered Simple lungs Lungs with Lungs with Lungs with
by an with small extensive
operculum, olds and olding to para-bronchial alveoli,
with one gill moist skin or increase the
slit gas exchange surace area tubes, ventilated
ventilated using
using air sacs ribs and a
diaphragm
No limbs Tetrapods with pentadactyl limbs
Fins Four legs Four legs (in Two legs and Four legs in
supported by when adult most species) two wings most (or two
rays legs and two
wings/arms)
Eggs and sperm released or Sperm passed into the emale or internal
external ertilization ertilization
Remain Larval stage Female lays Female lays Most give
in water that lives in eggs with sot eggs with hard birth to live
throughout water and shells shells young and
their lie cycle adult that all eed
usually lives Teeth all o Beak but no young with
on land one type, with teeth milk rom
no living parts mammary
Swim bladder Eggs coated glands
containing gas in protective
or buoyancy jelly Teeth o
dierent
types with a
living core
Do not maintain constant body temperature Maintain constant body
temperature
Table 6
5.4 clADistics
5.4 cad
udertdig applictio
A clade is a group o organisms that have Cladograms including humans and other
evolved rom a common ancestor. primates.
Evidence or which species are part o a clade Reclassifcation o the fgwort amily using
can be obtained rom the base sequences evidence rom cladistics.
o a gene or the corresponding amino acid
sequence o a protein. skill
Sequence dierences accumulate gradually Analysis o cladograms to deduce evolutionary
so there is a positive correlation between the relationships.
number o dierences between two species
and the time since they diverged rom a ntre of ciece
common ancestor.
Falsifcation o theories with one theory being
Traits can be analogous or homologous. superseded by another: plant amilies have
been reclassifed as a result o evidence rom
Cladograms are tree diagrams that show the cladistics.
most probable sequence o divergence in
clades.
Evidence rom cladistics has shown that
classifcations o some groups based
on structure did not correspond with the
evolutionary origins o a group o species.
Clde
A clade is a group o organisms that have evolved rom
a common ancestor.
Species can evolve over time and split to orm new species. This has
happened repeatedly with some highly successul species, so that
there are now large groups o species all derived rom a common
ancestor. These groups o species can be identifed by looking or shared
characteristics. A group o organisms evolved rom a common ancestor is
called a clade.
C lades include all the species alive today, together with the common
ancestral species and any species that evolved rom it and then became
extinct. They can be very large and include thousands o species, or
very small with j ust a ew. For example, birds orm one large clade with
about ten thousand living species because they have all evolved rom
a common ancestral species. The tree Ginkgo biloba is the only living
member o a clade that evolved about 270 million years ago. There have
been other species in this clade but all are now extinct.
269
5 Evolution and biodivErsity
Aciviy threatened or have close relatives. In some cases species
are the last members o a clade that has existed or tens
the EDGE of Exisence projec or hundreds o millions o years and it would be tragic or
them to become extinct as a result o human activities.
The aim o this project is to identiy animal species
that have ew or no close relatives and are thereore What species on EDGE lists are in your part o the world
members o very small clades. The conservation status and what can you do to help conserve them?
o these species is then assessed. Lists are prepared o
species that are both Evolutionarily Distinct and Globally http://www.edgeoexistence.org/species/
Endangered, hence the name o the project. Species
on these lists can then be targeted or more intense
conservation eforts than other species that are either not
Figure 1 Two species on the EDGE list: Loris tardigradus tardigradus (Horton Plains slender loris) rom Sri Lanka and Bradypus
pygmaeus (Pygmy three-toed sloth) rom Isla Escudo de Veraguas, a small island of the coast o Panama
Identifying members of a clade
Evidence or which species are part o a clade can be
obtained rom the base sequences o a gene or the
corresponding amino acid sequence o a protein.
It is not always obvious which species have evolved from a common
ancestor and should therefore be included in a clade.
The most objective evidence comes from base sequences of genes or
amino acid sequences of proteins. Species that have a recent common
ancestor can be expected to have few differences in base or amino acid
sequence. C onversely, species that might look similar in certain respects
but diverged from a common ancestor tens of millions of years ago are
likely to have many differences.
270
5.4 clADistics
Moleculr clocks
Sequence diferences accumulate gradually so there is
a positive correlation between the number o diferences
between two species and the time since they diverged
rom a common ancestor.
Dierences in the base sequence o DNA and thereore in the amino
acid sequence o proteins are the result o mutations. They accumulate
gradually over long periods o time. There is evidence that mutations
occur at a roughly constant rate so they can be used as a molecular
clock. The number o dierences in sequence can be used to deduce how
long ago species split rom a common ancestor.
For example, mitochondrial DNA rom three humans European
and our related primates has been completely Japanese
sequenced. From the dierences in base sequence, a African
hypothetical ancestry has been constructed. It is shown Common chimpanzee
in fgure 2. Using dierences in base sequence as a Pygmy chimpanzee (bonobo)
molecular clock, these approximate dates or splits Gorilla
between groups have been deduced: Oran -utan
70,000 years ago, EuropeanJapanese split
1 40,000 years ago, AricanEuropean/Japanese split
5,000,000 years ago, humanchimpanzee split Figure 2
anlogous nd homologous trits
Traits can be analogous or homologous.
Similarities between organisms can either be homologous or analogous.
Homologous structures are similar because o similar ancestry; or
example the chicken wing, human arm and other pentadactyl orelimbs.
Analogous structures are similar because o convergent evolution. The
human eye and the octopus eye show similarities in structure and
unction but they are analogous because they evolved independently.
Problems in distinguishing between homologous and analogous
structures have sometimes led to mistakes in classifcation in the past.
For this reason the morphology (orm and structure) o organisms is
now rarely used or identiying members o a clade and evidence rom
base or amino acid sequences is trusted more.
cornea
iris
lens
retina
photoreceptors
optic nerve
Figure 3 The human eye ( left) and the octopus eye (right) are analogous because they are
quite similar yet evolved independently
271
5 Evolution and biodivErsity
turtles Cladograms
lizards
snakes Cladograms are tree diagrams that show the most
birds probable sequence o divergence in clades.
non-avian
dinosaurs A cladogram is a tree diagram based on similarities and dierences between
crocodiles the species in a clade. Cladograms are almost always now based on base
or amino acid sequences. Computer programs have been developed that
ancestral species A calculate how species in a clade could have evolved with the smallest
ancestral species B number o changes o base or amino acid sequence. This is known as the
principle o parsimony and although it does not prove how a clade actually
ancestral species C evolved, it can indicate the most probable sequence o divergence in clades.
Figure 4 A cladogram showing the The branching points on cladograms are called nodes. Usually two clades
hypothesized relationship between birds and branch o at a node but sometimes there are three or more. The node
the traditional taxonomic group the reptiles represents a hypothetical ancestral species that split to orm two or more
species. Option B includes instructions or constructing cladograms rom
Activity base sequences using computer sotware.
Figure 5 shows an artists impression Figure 4 is an example o a cladogram or birds and reptiles. It has been
o two pterosaurs, which were the rst based on morphology, so that extinct groups can be included.
chordates to develop powered fight.
They were neither birds nor dinosaurs. Birds, non-avian dinosaurs and ancestral species A orm a clade
Where might pterosaurs have tted called dinosauria.
into the cladogram shown in gure 4?
Birds, non-avian dinosaurs, crocodiles and ancestral species B are
part o a clade called archosaurs.
Lizards, snakes and ancestral species C orm a clade called squamates.
This cladogram suggests either that birds should be regarded as reptiles
or that reptiles should be divided into two or more groups, as some
reptiles are more closely related to birds than to other reptiles.
Figure 5 Two pterosaurs in fight 45,000
Primate cladograms 4.5 Myr ago
Cladograms including humans and 27,000
other primates.
1 Myr ago
The closest relatives o humans are chimpanzees
and bonobos. The entire genome o these three 12,000
species has been sequenced giving very strong
evidence or the construction o a cladogram Bonobo Chimpanzee Human
(fgure 6) . The numbers on the cladogram are
estimates o population sizes and dates when Figure 6
splits occurred. These are based on a molecular
clock with a mutation rate o 1 0 9 yr 1.
Figure 7 is a cladogram or primates and the most
closely related other groups o mammal. Primates
are an order o mammals that have adaptations
or climbing trees. Humans, monkeys, baboons,
gibbons and lemurs are primates.
272
5.4 clADistics
anlysis of cldogrms Cavies and Coypu
Porcupines
Analysis o cladograms to deduce evolutionary Mice and Rats
relationships. Beavers
Chipmunks
The pattern o branching in a cladogram is assumed to match the Rabbits
evolutionary origins o each species. The sequence o splits at nodes is Primates
thereore a hypothetical sequence in which ancestors o existing clades Treeshrews
diverged. I two clades on a cladogram are linked at a node, they are
relatively closely related. I two species are only connected via a series Figure 7
o nodes, they are less closely related.
Avy
Some cladograms include numbers to indicate numbers o dierences
in base or amino acid sequence or in genes. Because genetic changes A adogram for he grea ape
are assumed to occur at a relatively constant rate, these numbers can
be used to estimate how long ago two clades diverged. This method The great apes are a amily o
o estimating times is called a molecular clock. Some cladograms primates. The taxonomic name is
are drawn to scale according to estimates o how long ago each split Hominidae. There are fve species
occurred. on Earth today, all o which are
decreasing in number apart rom
Although cladograms can provide strong evidence or the evolutionary humans. Figure 6 is a cladogram
history o a group, they cannot be regarded as proo. Cladograms are or three o the species. Use
constructed on the assumption that the smallest possible number this inormation to expand the
o mutations occurred to account or current base or amino acid cladogram to include all the great
sequence dierences. Sometimes this assumption is incorrect apes: the split between humans
and pathways o evolution were more convoluted. It is thereore and gorillas occurred about
important to be cautious in analysis o cladograms and where possible 10 million years ago and the split
compare several versions that have been produced independently between humans and orang-
using dierent genes. utans about 15 million years ago.
Daa-baed queon: Origins of turtles and lizards
Cladograms based on morphology suggest the short-tailed opossum or to the duck-billed
that turtles and lizards are not a clade. To test
platypus. [2]
this hypothesis, microRNA genes have been 2 Calculate how many microRNA genes are
compared or nine species o chordate. The
results were used to construct the cladogram in ound in the mammal clade on the cladogram
but not in the other clades. [2]
fgure 8. The numbers on the cladogram show 3 Discuss whether the evidence in the
which microRNA genes are shared by members
o a clade but not members o other clades. For cladogram supports the hypothesis that turtles
and lizards are not a clade. [3]
example, there are six microRNA genes ound in 4 Evaluate the traditional classifcation o
humans and short-tailed opossums but not in any o
the other chordates on the cladogram. tetrapod chordates into amphibians, reptiles,
1 Deduce, using evidence rom the cladogram, birds and mammals using evidence rom the
whether humans are more closely related to
cladogram. [3]
273
5 Evolution and biodivErsity
African clawed frog
6 Human
3 Short-tailed opossum
Duck-billed platypus
340
671
761
885
1251
1397
186
590
873
490 145119 Zebra nch
1397 1460 Chicken
1467
1559
1567
1641
1669
1729
1743
1744
1756
1759
1781
1784
1789
1803
2131
2954
2964
1791
1
Alligator
1
1677 4 Painted turtle
Lizard
5390
5391
5392
5393
Figure 8
Cladograms and reclassifcation
Evidence rom cladistics has shown that classifcations o
some groups based on structure did not correspond with
the evolutionary origins o a group o species.
The construction o cladograms based on base and amino acid sequences
only became possible towards the end o the 2 0th century. B eore that
the sequence data was not available and computer sotware had not
been developed to do the analysis. The construction o cladograms and
identifcation o clades is known as cladistics.
Cladistics has caused some revolutions in plant and animal
classifcation. It is now clear rom cladograms that traditional
classifcation based on morphology does not always match the
evolutionary origins o groups o species. As a result some groups have
been reclassifed. Some groups have been merged, others have been
divided and in some cases species have been transerred rom one
group to another.
Reclassifcation o groups o organisms is time-consuming and
potentially disruptive or biologists, but it is certainly worthwhile. The
new classifcations based on cladistics are likely to be much closer to
a truly natural classifcation so their predictive value will be higher.
They have revealed some unnoticed similarities between groups and
also some signifcant dierences between species previously assumed
to be similar.
274
5.4 clADistics
Cladograms and alsifcation
Falsifcation o theories with one theory being
superseded by another: plant amilies have been
reclassifed as a result o evidence rom cladistics.
The reclassifcation o plants on the basis o discoveries in cladistics
is a good example o an important process in science: the testing o
theories and o replacement o theories ound to be alse with new
theories. The classifcation o angiospermophytes into amilies based
on their morphology was begun by the French botanist Antoine
Laurent de Jussieu in Genera plantarum, published in 1 789 and
revised repeatedly during the 1 9th century.
Classifcation o the fgwort amily
Reclassifcation o the fgwort amily using evidence rom cladistics.
There are more than 400 amilies o angiosperms. Taxonomists recently investigated the
Until recently the eighth largest was the evolutionary origins o the fgwort amily
Scrophulariaceae, commonly known as the using cladistics. One important research project
fgwort amily. It was one o the original amilies compared the base sequences o three chloroplast
proposed by de Jussieu in 1 789. He gave it the genes in a large number o species in genera
name Scrophulariae and included sixteen genera, traditionally assigned to the Scrophulariaceae and
based on similarities in their morphology. As genera in closely related amilies. It was ound
more plants were discovered, the amily grew that species in the fgwort amily were not a true
until there were over 275 genera, with more than clade and that fve clades had incorrectly been
5,000 species. combined into one amily.
Two small families were merged
with the gwort family:
the buddleja family, Buddlejaceae
and the myoporum family, Myoporaceae
Two genera were moved to The gwort Nearly fty genera have
a newly-created family, family been moved to the
the calceolaria family, plantain family,
Calceolariaceae Scrophulariaceae Plantaginaceae
Thirteen genera have been About twelve genera of
transferred to a newly-created parasitic plants have been
moved to the broomrape
family, the lindernia family,
Linderniaceae family, Orobanchaceae
Figure 9
275
5 Evolution and biodivErsity
A major reclassifcation has now been carried out. changes is shown in fgure 9. This reclassifcation
Less than hal o the species have been retained has been welcomed as it was widely appreciated
in the amily, which is now only the thirty-sixth beore that the Scrophulariaceae had been a rag-bag
largest among the angiosperms. A summary o the o species rather than a natural group.
Figure 10 Antirrhinum majus has been transerred rom the Figure 11 Scrophularia peregrina has remained in the
fgwort amily to the plantain amily fgwort amily
276
QuEstions
Questions% increase in algal volume 4 Which o the ollowing processes are required or
copper tolerance to develop in a population?
The bar charts in fgure 1 2 show the growth o (i) variation in copper tolerance
three populations o an alga, Ectocarpus siliculosus, (ii) inheritance o copper tolerance
at dierent copper concentrations. One population (iii) ailure o algae with lower copper
came rom an unpolluted environment at tolerance to survive or reproduce.
Rhosneigr in Wales. The other two came rom the a) i) only
undersides o ships that had been painted with a b) i) and ii) only
copper-containing anti-ouling paint. c) i) and iii) only
d) i) , ii) and iii) .
500 Rhosneigr
0
M.V. San Nicholas
500
0 5 In fgure 1 3, each number represents a
M.V. Amama species. The closer that two numbers are on
the diagram the more similar the two species.
500 The circles represent taxonomic groups. For
example, the diagram shows that 2, 3, 4 and
0 5 are in the same genus.
0.0 0.01 0.05 0.1 0.5 1.0 5.0 10.0
concentration of copper ( mg dm-3)
Figure 12
1 How much higher was the maximum copper 1 23 34
concentration tolerated by the algae rom 45
ships than the algae rom an unpolluted 67
environment?
a) 0.09 times higher b) 0.1 1 times higher 8
c) 1 .0 times higher d) 1 0 times higher. 9 10 11311142
15 16
19 17 18 24 25
20 21 26 27
2 What is the reason or results lower than zero 28 29
on the bar charts? 22
23 30
a) The volume o algae decreased.
31 32
b) The algae all died. 33
c) Increases in volume were less than 1 00%. Figure 13
d) Results were too small to measure a) State one species that is in a genus [1 ]
a c c u r a te l y. with no other species.
b) State the species that are in a amily [2]
with two genera.
3 What was the reason or the dierence in c) State the species that are in an order [2]
copper tolerance between the algae? with two amilies.
a) The algae on the ships absorbed copper. d) State the species that are in a class with
b) The algae can develop copper tolerance and three orders. [2]
pass it on to their ospring.
e) Deduce whether species 8 is more closely
c) The copper in the paint caused mutations. related to species 1 6 or species 6.
d) The copper in the paint caused natural f) Explain why three concentric circles have
selection or higher levels o copper tolerance.
been drawn around species 34 on the
diagram. [2]
277
51 E vo l u t i o n an d b i o d i vE r s i t y
6 The map in gure 1 4 shows the distribution Key
in the 1 95 0s o two orms o Biston betularia Non-melanic
in B ritain and Ireland. Biston betularia is a Melanic
species o moth that fies at night. It spends
the daytime roosting on the bark o trees. The
non-melanic orm has white wings, peppered
with black spots. The melanic orm has black
wings. Beore the industrial revolution, the
melanic orm was very rare. The prevailing
wind direction is rom the Atlantic Ocean, to
the west.
a) State the maximum and minimum [2]
percentages o the melanic orm.
b) Outline the trends in the distribution o
the two orms o Biston betularia, shown
in gure 1 4. [2]
c) Explain how natural selection can cause
moths such as Biston betularia to develop
camoufaged wing markings. [4] Figure 14
d) Suggest reasons or the distribution o
the two orms. [2]
278
6 HUmAN pHySIology
Intrductin products. The skin and immune system resist the
continuous threat of invasion by pathogens. The
Research into human physiology is the lungs are actively ventilated to ensure that gas
foundation of modern medicine. Body functions exchange can occur passively. Neurons transmit
are carried out by specialized organ systems. the message, synapses modulate the message.
The structure of the wall of the small intestine Hormones are used when signals need to be
allows it to move, digest and absorb food. The widely distributed.
blood system continuously transports substances
to cells and simultaneously collects waste
6.1 Digestion and absorption
Understandin Aicatins
The contraction o circular and longitudinal Processes occurring in the small intestine that
muscle layers o the small intestine mixes the result in the digestion o starch and transport o
ood with enzymes and moves it along the gut. the products o digestion to the liver.
The pancreas secretes enzymes into the lumen Use o dialysis tubing to model absorption o
o the small intestine. digested ood in the intestine.
Enzymes digest most macromolecules in ood Skis
into monomers in the small intestine.
Production o an annotated diagram o the
Villi increase the surace area o epithelium digestive system.
over which absorption is carried out.
Identication o tissue layers in transverse
Villi absorb monomers ormed by digestion as sections o the small intestine viewed with a
well as mineral ions and vitamins. microscope or in a micrograph.
Diferent methods o membrane transport are Nature f science
required to absorb diferent nutrients.
Use models as representations o the real
world: dialysis tubing can be used to model
absorption in the intestine.
279
61 H u mC EaLnLpBHIyOsLiOoGlYo g y
Structure of the digestive system
Production of an annotated diagram of the digestive system.
The part of the human body used for digestion Surfactants and other enzymes are secreted
can be described in simple terms as a tube by accessory glands that have ducts leading
through which food passes from the mouth to to the digestive system. Controlled, selective
the anus. The role of the digestive system is to absorption of the nutrients released by digestion
break down the diverse mixture of large carbon takes place in the small intestine and colon, but
compounds in food, to yield ions and smaller some small molecules, notably alcohol, diffuse
compounds that can be absorbed. For proteins, through the stomach lining before reaching the
lipids and polysaccharides digestion involves small intestine.
several stages that occur in different parts of
the gut. Figure 1 is a diagram of the human digestive
system. The part of the esophagus that passes
Digestion requires surfactants to break up lipid through the thorax has been omitted. This
droplets and enzymes to catalyse reactions. diagram can be annotated to indicate the
Glandular cells in the lining of the stomach functions of different parts. A summary of
and intestines produce some of the enzymes. functions is given in table 1 below.
mouth Structure Function
Mouth
esophagus Voluntary control of eating and
Esophagus swallowing. Mechanical digestion
gall bladder Stomach of food by chewing and mixing with
liver saliva, which contains lubricants and
stomach Small intestine enzymes that start starch digestion
pancreas
small intestine Pancreas Movement of food by peristalsis
Liver from the mouth to the stomach
large intestine Gall bladder
anus Large intestine Churning and mixing with secreted
water and acid which kills foreign
Figure 1 The human digestive system Table 1 bacteria and other pathogens in
food, plus initial stages of protein
digestion
Final stages of digestion of lipids,
carbohydrates, proteins and nucleic
acids, neutralizing stomach acid,
plus absorption of nutrients
Secretion of lipase, amylase and
protease
Secretion of surfactants in bile to
break up lipid droplets
Storage and regulated release of bile
Re-absorption of water,
further digestion especially of
carbohydrates by symbiotic
bacteria, plus formation and storage
of feces
280
6.1 DigeStion anD abSorption
Structure of the wall of the small intestine
Identifcation o tissue layers in transverse sections o the small intestine viewed
with a microscope or in a micrograph.
The wall o the small intestine is made o layers
o living tissues, which are usually quite easy
to distinguish in sections o the wall. From the
outside o the wall going inwards there are
our layers:
serosa an outer coat
muscle layers longitudinal muscle and inside
it circular muscle
sub-mucosa a tissue layer containing blood
and lymph vessels
mucosa the lining o the small intestine, Figure 2 Longitudinal section through the wall o the small
with the epithelium that absorbs nutrients on intestine. Folds are visible on the inner surace and on
its inner surace. these olds are fnger-like projections called villi. All o the
our main tissue layers are visible, including both circular
and longitudinal parts o the muscle layer. The mucosa is
stained darker than the sub-mucosa
peristalsis acvy
The contraction o circular and longitudinal muscle layers tssu l dms f h
o the small intestine mixes the ood with enzymes and s wll
moves it along the gut.
To practice your skill at
The circular and longitudinal muscle in the wall o the gut is identiying tissue layers,
smooth muscle rather than striated muscle. It consists o relatively short draw a plan diagram o the
cells, not elongated fbres. It oten exerts continuous moderate orce, tissues in the longitudinal
interspersed with short periods o more vigorous contraction, rather section o the intestine wall
than remaining relaxed unless stimulated to contract. in fgure 2. To test your skill
urther, draw a plan diagram
Waves o muscle contraction, called peristalsis, pass along the intestine. to predict how the tissues
Contraction o circular muscles behind the ood constricts the gut to o the small intestine would
prevent it rom being pushed back towards the mouth. Contraction o appear in a transverse
longitudinal muscle where the ood is located moves it on along the gut. section.
The contractions are controlled unconsciously not by the brain but by
the enteric nervous system, which is extensive and complex.
Swallowed ood moves quickly down the esophagus to the stomach in
one continuous peristaltic wave. Peristalsis only occurs in one direction,
away rom the mouth. When ood is returned to the mouth rom the
stomach during vomiting, abdominal muscles are used rather than the
circular and longitudinal muscle in the gut wall.
In the intestines the ood is moved only a ew centimetres at a time so
the overall progression through the intestine is much slower, allowing
time or digestion. The main unction o peristalsis in the intestine is
churning o the semi-digested ood to mix it with enzymes and thus
speed up the process o digestion.
281
16 H u mC EaLnLpBHIyOsLiOoGlYo g y
Figure 3 Three-dimensional image showing pancreatic juice
the wave of muscle contraction (brown) in the
esophagus during swallowing. Green indicates The pancreas secretes enzymes into the lumen of the
when the muscle is exerting less force. Time small intestine.
is shown left to right. At the top the sphincter
between the mouth and the esophagus is The pancreas contains two types o gland tissue. Small groups o cells secrete
shown permanently constricted apart from a the hormones insulin and glucagon into the blood. The remainder o the
brief opening when swallowing starts pancreas synthesizes and secretes digestive enzymes into the gut in response
to eating a meal. This is mediated by hormones synthesized and secreted
by the stomach and also by the enteric nervous system. The structure o
the tissue is shown in fgure 4. Small groups o gland cells cluster round the
ends o tubes called ducts, into which the enzymes are secreted.
The digestive enzymes are synthesized in pancreatic gland cells on ribosomes
on the rough endoplasmic reticulum. They are then processed in the Golgi
apparatus and secreted by exocytosis. Ducts within the pancreas merge into
larger ducts, fnally orming one pancreatic duct, through which about a litre
o pancreatic juice is secreted per day into the lumen o the small intestine.
Pancreatic juice contains enzymes that digest all the three main types o
macromolecule ound in ood:
amylase to digest starch
lipases to digest triglycerides, phospholipids
proteases to digest proteins and peptides.
secretory vesicles Digestion in the small intestine
one acinus Enzymes digest most macromolecules in food into
monomers in the small intestine.
The enzymes secreted by the pancreas into the lumen o the
small intestine carry out these hydrolysis reactions:
secretory cells basement membrane starch is digested to maltose by amylase
wall of duct triglycerides are digested to atty acids and glycerol or atty
acids and monoglycerides by lipase
lumen of duct
phospholipids are digested to atty acids, glycerol and
Figure 4 Arrangement of cells and ducts in a part of phosphate by phospholipase
the pancreas that secretes digestive enzymes
proteins and polypeptides are digested to shorter peptides by
protease.
This does not complete the process o digestion into molecules small
enough to be absorbed. The wall o the small intestine produces
a variety o other enzymes, which digest more substances. Some
enzymes produced by gland cells in the intestine wall may be secreted
in intestinal juice but most remain immobilized in the plasma
membrane o epithelium cells lining the intestine. They are active
there and continue to be active when the epithelium cells are abraded
o the lining and mixed with the semi-digested ood.
Nucleases digest DNA and RNA into nucleotides.
Maltase digests maltose into glucose.
282
6.1 DigeStion anD abSorption
Lactase digests lactose into glucose and galactose.
Sucrase digests sucrose into glucose and ructose.
Exopeptidases are proteases that digest peptides by removing single
amino acids either rom the carboxy or amino terminal o the chain
until only a dipeptide is let.
Dipeptidases digest dipeptides into amino acids.
Because o the great length o the small intestine, ood takes hours to Figure 5 Cystic fbrosis causes the pancreatic
pass through, allowing time or digestion o most macromolecules to duct to become blocked by mucus. Pills
be completed. Some substances remain largely undigested, because containing synthetic enzymes help digestion in
humans cannot synthesize the necessary enzymes. Cellulose or example the small intestine. The photograph shows one
is not digested and passes on to the large intestine as one o the main days supply or a person with cystic fbrosis
components o dietary fbre.
Villi and the surface area for digestion
Villi increase the surface area of epithelium over which
absorption is carried out.
The process o taking substances into cells and the blood is called
absorption. In the human digestive system nutrients are absorbed epithelium
principally in the small intestine. The rate o absorption depends on
the surace area o the epithelium that carries out the process. The
small intestine in adults is approximately seven metres long and layer of microvilli lacteal (a branch
ofthe lymphatic
2 5 3 0 millimetres wide and there are olds on its inner surace, giving on surface of system)
a large surace area. This area is increased by the presence o villi. epithelium
Villi are small fnger-like proj ections o the mucosa on the inside o the blood capillary
intestine wall. A villus is between 0.5 and 1 .5 mm long and there can
be as many as 40 o them per square millimetre o small intestine wall.
They increase the surace area by a actor o about 1 0.
Absorption by villi goblet cells
(secrete mucus)
Villi absorb monomers formed by digestion as well as Figure 6 Structure o an intestinal villus
mineral ions and vitamins.
The epithelium that covers the villi must orm a barrier to harmul
substances, while at the same time being permeable enough to allow
useul nutrients to pass through.
Villus cells absorb these products o digestion o macromolecules in ood:
glucose, ructose, galactose and other monosaccharides
any o the twenty amino acids used to make proteins
atty acids, monoglycerides and glycerol
bases rom digestion o nucleotides.
They also absorb substances required by the body and present in oods
but not needing digestion:
mineral ions such as calcium, potassium and sodium Figure 7 Scanning electron micrograph o villi
vitamins such as ascorbic acid (vitamin C) . in the small intestine
283
61 H u mC EaLnLpBHIyOsLiOoGlYo g y
Some harmul substances pass through the epithelium and are
subsequently removed rom the blood and detoxied by the liver. S ome
harmless but unwanted substances are also absorbed, including many
o those that give ood its colour and favour. These pass out in urine.
Small numbers o bacteria pass through the epithelium but are quickly
removed rom the blood by phagocytic cells in the liver.
methods of absorption
Diferent methods o membrane transport are required to
absorb diferent nutrients.
To be absorbed into the body, nutrients must pass rom the lumen o
the small intestine to the capillaries or lacteals in the villi. The nutrients
must rst be absorbed into epithelium cells through the exposed
part o the plasma membrane that has its surace area enlarged with
microvilli. The nutrients must then pass out o this cell through the
plasma membrane where it aces inwards towards the lacteal and blood
capillaries o the villus.
Many dierent mechanisms move nutrients into and out o the villus
epithelium cells: simple diusion, acilitated diusion, active transport
and exocytosis. These methods can be illustrated using two dierent
examples o absorption: triglycerides and glucose.
Triglycerides must be digested beore they can be absorbed. The
products o digestion are atty acids and monoglycerides, which can
be absorbed into villus epithelium cells by simple diusion as they
can pass between phospholipids in the plasma membrane.
Fatty acids are also absorbed by acilitated diusion as there are atty
acid transporters, which are proteins in the membrane o the microvilli.
Once inside the epithelium cells, atty acids are combined with
monoglycerides to produce triglycerides, which cannot diuse back
out into the lumen.
lumen of villus epithelium interior
small intestine of villus
Na+ low Na+ 3Na+ blood
concentration capillary
glucose 2K+
fatty acids and glucose
monoglycerides
lacteal
triglyceride lipoprotein
Figure 8 Methods of absorption in the small intestine
284
6.1 DigeStion anD abSorption
Triglycerides coalesce with cholesterol to orm droplets with a
diameter o about 0.2 m, which become coated in phospholipids
and protein.
These lipoprotein particles are released by exocytosis through the
plasma membrane on the inner side o the villus epithelium cells.
They then either enter the lacteal and are carried away in the lymph,
or enter the blood capillaries in the villi.
Glucose cannot pass through the plasma membrane by simple
diusion because it is polar and thereore hydrophilic.
Sodiumpotassium pumps in the inwards-acing part o the plasma
membrane pump sodium ions by active transport rom the cytoplasm
to the interstitial spaces inside the villus and potassium ions in the
opposite direction. This creates a low concentration o sodium ions
inside villus epithelium cells.
Sodiumglucose co-transporter proteins in the microvilli transer
a sodium ion and a glucose molecule together rom the intestinal
lumen to the cytoplasm o the epithelium cells. This type o
acilitated diusion is passive but it depends on the concentration
gradient o sodium ions created by active transport.
Glucose channels allow the glucose to move by acilitated diusion
rom the cytoplasm to the interstitial spaces inside the villus and on
into blood capillaries in the villus.
Starch digestion in the small intestine
Processes occurring in the small intestine that result in the digestion of starch and
transport of the products of digestion to the liver.
Starch digestion illustrates some important CH2OH CH2OH
processes including catalysis, enzyme specifcity
and membrane permeability. S tarch is a O O
macromolecule, composed o many -glucose OH OH
monomers linked together in plants by
condensation reactions. It is a major constituent OH O O CH2OH CH2OH
o plant-based oods such as bread, potatoes and OH
pasta. Starch molecules cannot pass through OH O O
membranes so must be digested in the small CH2OH OH OH
intestine to allow absorption. O CH2
O
All o the reactions involved in the digestion o OH
starch are exothermic, but without a catalyst they OH
happen at very slow rates. There are two types o
molecule in starch: OH O O O O
OH OH OH OH
amylose has unbranched chains o -glucose
linked by 1 ,4 bonds; Figure 9 Small portion of an amylopectin molecule showing
six -glucose molecules, all linked bv 1,4 bonds apart from
amylopectin has chains o -glucose linked one 1,6 bond that creates a branch
by 1 ,4 bonds, with some 1 ,6 bonds that make
the molecule branched. The enzyme that begins the digestion o both
orms o starch is amylase. Saliva contains
amylase but most starch digestion occurs in the
small intestine, catalysed by pancreatic amylase.
Any 1 ,4 bond in starch molecules can be broken
by this enzyme, as long as there is a chain o at
least our glucose monomers. Amylose is thereore
285
61 H u mC EaLnLpBHIyOsLiOoGlYo g y
digested into a mixture o two- and three-glucose capillaries close to the epithelium ensures that
ragments called maltose and maltotriose. glucose only has to travel a short distance to
enter the blood system. Capillary walls consist o
Because o the specicity o its active site, amylase a single layer o thin cells, with pores between
cannot break 1 ,6 bonds in amylopectin. Fragments adjacent cells, but these capillaries have larger
o the amylopectin molecule containing a pores than usual, aiding the entry o glucose.
1 ,6 bond that amylase cannot digest are called
dextrins. Digestion o starch is completed by Blood carrying glucose and other products o
three enzymes in the membranes o microvilli digestion fows though villus capillaries to venules
on villus epithelium cells. Maltase, glucosidase in the sub-mucosa o the wall o the small
and dextrinase digest maltose, maltotriose and intestine. The blood in these venules is carried
dextrins into glucose. via the hepatic portal vein to the liver, where
excess glucose can be absorbed by liver cells and
Glucose is absorbed into villus epithelium cells converted to glycogen or storage. Glycogen is
by co-transport with sodium ions. It then moves similar in structure to amylopectin, but with
by acilitated diusion into the fuid in interstitial more 1 ,6 bonds and thereore more extensive
spaces inside the villus. The dense network o branching.
modelling physiological processes
Use models as representations of the real world: dialysis tubing can be used
to model absorption in the intestine.
Living systems are complex and when
experiments are done on them, many actors can
infuence the results. It can be very dicult to
control all o the variables and analysis o results
becomes dicult. Sometimes it is better to carry
out experiments using only parts o systems. For
example, much research in physiology has been
carried out using clones o cells in tissue culture
rather than whole organisms.
Another approach is to use a model to represent Figure 10 The Dynamic Gastric Model with its inventor, Richard
part o a living system. B ecause it is much simpler, Faulks, adjusting the antrum mechanism
a model can be used to investigate specic aspects
o a process. A recent example is the Dynamic mimic the wall o the gut, which is also more
Gastric Model, a computer-controlled model o permeable to small rather than large particles.
the human stomach that carries out mechanical Dialysis tubing can be used to model absorption
and chemical digestion o real ood samples. It can by passive diusion and by osmosis. It cannot
be used to investigate the eects o diet, drugs, model active transport and other processes that
alcohol and other actors on digestion. occur in living cells
A simpler example is the use o dialysis tubing
made rom cellulose. Pores in the tubing allow
water and small molecules or ions to pass through
reely, but not large molecules. These properties
286
6.1 DigeStion anD abSorption
modelling the sall intestine
Use of dialysis tubing to model absorption of digested food in the intestine.
To make a model o the small intestine, cut a Suggest improvements to the method, or suggest
length o dialysis tubing and seal one end by tying an entirely dierent method o investigating the
a knot in the tubing or tying with a piece o cotton need or digestion.
thread. Pour in a suitable mixture o oods and
seal the open end by tying with a piece o cotton 2 Investigating membrane permeability using
thread. Two experiments using model intestines a model of the small intestine
made in this way are suggested here:
Cola drinks contain a mixture o substances
1 Investigating the need for digestion using with dierent particle sizes. They can be used
a model of the small intestine to represent ood in the small intestine. Dialysis
tubing is semi-permeable so can be used to model
Set up the apparatus shown in gure 1 1 and leave the wall o the small intestine.
it or one hour.
Predictions
Results
Cola contains glucose, phosphoric acid and
To obtain the results or the experiment, take caramel, a complex carbohydrate added to
the bags out o each tube, open them and pour produce a brown colour. Predict which o these
the solutions rom them into separate test tubes substances will diuse out o the bag, with reasons
rom the liquids in the tubes. You should now or your predictions. Predict whether the bag will
have our samples o fuid. Divide each o these gain or lose mass during the experiment.
samples into two halves and test one hal or
starch and the other hal or sugars. I n s tru cti o n s
1 Make the model intestine with cola inside.
2 Rinse the outside o the bag to wash o any
traces o cola and then dry the bag.
10 ml 10 ml of cola, left to go at tube
of 1% 1% starch before being put top of bag sealed
starch solution with cotton thread
solution and 1 ml into the tube
and 1 ml of 1% dialysis tubing
of water amylase pure water
solution minimum volume base of bag knotted
to surround the bag to prevent leaks
water
maintained
at 40C
water bags made water
of dialysis
(Visking) tubing
Figure 11 Apparatus for showing the need for digestion
Record all the results in the way that you think is spotting
most appropriate. tile
Conclusions and evaluation pH indicator
State careully all the conclusions that you can Figure 12 Apparatus for membrane permeability experiment
make rom your results.
287
Discuss the strengths and weaknesses o this
method o investigating the need or digestion.
61 H u mC EaLnLpBHIyOsLiOoGlYo g y
3 Find the mass o the bag using an electronic vary or these test strips. Follow the
balance. instructions and work out the glucose
concentration o the water.
4 When you are ready to start the experiment,
place the bag in pure water in a test tube. 6 Ater testing the water or the last time,
remove the bag, dry it and fnd its mass again
5 Test the water around the bag at suitable time with the electronic balance.
intervals. A suggested range is 1 , 2, 4, 8 and
1 6 minutes. At each time lit the bag up and Conclusions
down a ew times to mix the water in the
tube, then do these tests: a) Explain the conclusions that you can draw
Look careully at the water to see whether about the permeability o the dialysis tubing
it is still clear or has become brown.
rom the tests o the water and rom the
Use a dropping pipette to remove a ew
drops o the water and test them in a change in mass o the bag. [5]
spotting tile with a narrow-range pH
indicator. Use a colour chart to work out b) Compare and contrast the dialysis tubing
the pH.
and the plasma membranes that carry out
Dip a glucose test strip into the water and
record the colour that it turns. Instructions absorption in villus epithelium cells in the
wall o the intestine. [5]
c) Use the results o your experiment to predict
the direction o movement o water by
osmosis across villus epithelium cells. [5]
TOK
What are some o the variables that afect perspectives as to what is normal?
In some adult humans, levels o lactase are too low continue to consume milk into adulthood are thereore
to digest lactose in milk adequately. Instead, lactose unusual. Inability to consume milk because o lactose
passes through the small intestine into the large intolerance should not thereore be regarded as abnormal.
intestine, where bacteria eed on it, producing carbon
dioxide, hydrogen and methane. These gases cause The second argument is a simple mathematical one: a
some unpleasant symptoms, discouraging consumption high proportion o humans are lactose intolerant.
o milk. The condition is known as lactose intolerance. It
has sometimes in the past been regarded as an abnormal The third argument is evolutionary. Our ancestors were
condition, or even as a disease, but it could be argued almost certainly all lactose intolerant, so this is the
that lactose intolerance is the normal human condition. natural or normal state. Lactose tolerance appears
to have evolved separately in at least three centres:
The rst argument or this view is a biological one. Female Northern Europe, parts o Arabia, the Sahara and eastern
mammals produce milk to eed their young ofspring. Sudan, and parts o East Arica inhabited by the Tutsi and
When a young mammal is weaned, solid oods replace Maasai peoples. Elsewhere, tolerance is probably due to
milk and lactase secretion declines. Humans who migration rom these centres.
288
6.2 the blooD SyStem
6.2 t d ss
Understanding Applications
Arteries convey blood at high pressure rom the William Harveys discovery o the circulation o
ventricles to the tissues o the body. the blood with the heart acting as the pump.
Arteries have muscle and elastic bres in Causes and consequences o occlusion o the
their walls. coronary arteries.
The muscle and elastic bres assist in Pressure changes in the let atrium, let
maintaining blood pressure between pump ventricle and aorta during the cardiac cycle.
cycles.
Skills
Blood fows through tissues in capillaries
with permeable walls that allow exchange o Identication o blood vessels as arteries,
materials between cells in the tissue and the capillaries or veins rom the structure o
blood in the capillary. their walls.
Veins collect blood at low pressure rom the Recognition o the chambers and valves o
tissues o the body and return it to the atria o the heart and the blood vessels connected
the heart. to it in dissected hearts or in diagrams o
heart structure.
Valves in veins and the heart ensure circulation
o blood by preventing backfow. Nature of science
There is a separate circulation or the lungs. Theories are regarded as uncertain: William
Harvey overturned theories developed by the
The heartbeat is initiated by a group o ancient Greek philosopher Galen on movement
specialized muscle cells in the right atrium o blood in the body.
called the sinoatrial node.
The sinoatrial node acts as a pacemaker.
The sinoatrial node sends out an electrical
signal that stimulates contraction as it is
propagated through the walls o the atria and
then the walls o the ventricles.
The heart rate can be increased or
decreased by impulses brought to the
heart through two nerves rom the medulla
o the brain.
Epinephrine increases the heart rate to prepare
or vigorous physical activity.
289
16 H u mC EaLnLpBHIyOsLiOoGlYo g y
William Harvey and the circulatin f bld
William Harveys discovery of the circulation of the blood with the heart acting
as the pump.
William Harvey is usually credited with the published his theory about the circulation o blood
discovery o the circulation o the blood as in 1 628. It was not until 1 660, ater his death,
he combined earlier discoveries with his own that blood was seen fowing rom arteries to veins
research ndings to produce a convincing overall though capillaries as he had predicted.
theory or blood fow in the body. He overcame
widespread opposition by publishing his results
and also by touring Europe to demonstrate
experiments that alsied previous theories and
provided evidence or his theory. As a result his
theory became generally accepted.
Harvey demonstrated that blood fow through
the larger vessels is unidirectional, with valves
to prevent backfow. He also showed that the
rate o fow through major vessels was ar too
high or blood to be consumed in the body ater
being pumped out by the heart, as earlier theories
proposed. It must thereore return to the heart
and be recycled. Harvey showed that the heart
pumps blood out in the arteries and it returns in
veins. He predicted the presence o numerous ne
vessels too small to be seen with contemporary
equipment that linked arteries to veins in the
tissues o the body.
Blood capillaries are too narrow to be seen with Figure 1 Harveys experiment to demonstrate that blood fow
the naked eye or with a hand lens. Microscopes in the veins o the arm is unidirectional
had not been invented by the time that Harvey
overturning ancient theries in science
Theories are regarded as uncertain: William Harvey overturned theories developed
by the ancient Greek philosopher Galen on movement of blood in the body.
During the Renaissance, interest was reawakened vital spirits are distributed to the body by the
in the classical writings o Greece and Rome. This arteries. Some o the vital spirits fow to the brain,
stimulated literature and the arts, but in some to be converted into animal spirits, which are
ways it hampered progress in science. It became then distributed by the nerves to the body.
almost impossible to question the doctrines o
such writers as Aristotle, Hippocrates, Ptolemy William Harvey was unwilling to accept these
and Galen. doctrines without evidence. He made careul
observations and did experiments, rom which
According to Galen, blood is ormed in the liver he deduced that blood circulates through the
and is pumped to and ro between the liver and pulmonary and systemic circulations. He predicted
the right ventricle o the heart. A little blood the existence o capillaries, linking arteries and
passes into the let ventricle, where it meets air veins, even though the lenses o the time were
rom the lungs and becomes vital spirits. The not powerul enough or him to see them.
290
6.2 the blooD SyStem
The ollowing extract is rom Harveys book On the others: without which no one can properly
Generation of Animals, published in 1 65 1 when he become a student of any branch of natural
was 73. science. I would not have you therefore,
gentle reader, to take anything on trust
And hence it is that without the due from me concerning the Generation of
admonition of the senses, without frequent Animals: I appeal to your own eyes as
observation and reiterated experiment, my witness and judge. The method of
our mind goes astray after phantoms pursuing truth commonly pursued at this
and appearances. Diligent observation is time therefore is to be held erroneous and
therefore requisite in every science, and almost foolish, in which so many enquire
the senses are frequently to be appealed what things others have said, and omit
to. We are, I say, to strive after personal to ask whether the things themselves be
experience, not to rely of the experience of actually so or not.
Arteries acivi
Arteries convey blood at high pressure rom the ventricles Discussin qusins n
to the tissues o the body. Wii hrvs ds
Arteries are vessels that convey blood rom the heart to the tissues o 1 William Harvey reused
the body. The main pumping chambers o the heart are the ventricles. to accept doctrines
They have thick strong muscle in their walls that pumps blood into the without evidence. Are
arteries, reaching a high pressure at the peak o each pumping cycle. there academic contexts
The artery walls work with the heart to acilitate and control blood fow. where it is reasonable to
Elastic and muscle tissue in the walls are used to do this. accept doctrines on the
basis o authority rather
Elastic tissue contains elastin bres, which store the energy that stretches than evidence gathered
them at the peak o each pumping cycle. Their recoil helps propel the rom primary sources?
blood on down the artery. C ontraction o smooth muscle in the artery
wall determines the diameter o the lumen and to some extent the 2 Harvey welcomed
rigidity o the arteries, thus controlling the overall fow through them. questions and criticisms
o his theories when
Both the elastic and muscular tissues contribute to the toughness o the teaching anatomy
walls, which have to be strong to withstand the constantly changing and classes. Suggest why he
intermittently high blood pressure without bulging outwards (aneurysm) might have done this.
or bursting. The bloods progress along major arteries is thus pulsatile, not
continuous. The pulse refects each heartbeat and can easily be elt in arteries 3 Can you think oexamples
that pass near the body surace, including those in the wrist and the neck. othe phantoms and
appearances that Harvey
Each organ o the body is supplied with blood by one or more arteries. reers to?
For example, each kidney is supplied by a renal artery and the liver by
the hepatic artery. The powerul, continuously active muscles o the 4 Why does Harvey
heart itsel are supplied with blood by coronary arteries. recommend reiteration
o experiments?
Artery walls
5 Harvey practised as
Arteries have muscle and elastic fbres in their walls. a doctor, but ater the
publication in 1628 o
The wall o the artery is composed o several layers: his work on the
circulation o the blood,
tunica externa a tough outer layer o connective tissue ar ewer patients
consulted him. Why
tunica media a thick layer containing smooth muscle and elastic might this have been?
bres made o the protein elastin
tunica intima a smooth endothelium orming the lining o the artery.
291
16 H u mC EaLnLpBHIyOsLiOoGlYo g y
tunica externa tunica media
lumen tunica
intima (endothelium)
Figure 3 Structure of an artery
activity Figure 2 The cardiovascular system. The main artery that supplies oxygenated blood to
the tissues of the body is the aorta, shown as the red vessel that emerges from the heart
mesuring blood pressures and forms an arch with branches carrying blood to the arms and head. The aorta continues
Because arteries are through the thorax and abdomen, with branches serving the liver, kidneys, intestines and
distensible, blood pressure other organs
in those that pass near
the body surace can be Arterial blood pressure
measured relatively easily.
A common method is to The muscle and elastic bres assist in maintaining
infate an arm cu until it blood pressure between pump cycles.
squeezes the tissues (skin,
supercial at as well as The blood entering an artery rom the heart is at high pressure. The peak
the vessels themselves) pressure reached in an artery is called the systolic pressure. It pushes the
enough to stop blood wall o the artery outwards, widening the lumen and stretching elastic
fow. The pressure is then bres in the wall, thus storing potential energy.
released slowly until fow
resumes and the operator At the end o each heartbeat the pressure in the arteries alls suciently
or instrument can hear the or the stretched elastic bres to squeeze the blood in the lumen. This
pulse again. The pressures at mechanism saves energy and prevents the minimum pressure inside
which blood fow stops and the artery, called the diastolic pressure, rom becoming too low. B ecause
resumes are the systolic and it is relatively high, blood fow in the arteries is relatively steady and
diastolic pressures. They are continuous although driven by a pulsating heart.
measured with a pressure
monitor. According to the The circular muscles in the wall o the artery orm a ring so when they
American Heart Association contract, in a process called vasoconstriction, the circumerence is reduced
the desired blood pressures and the lumen is narrowed. Vasoconstriction increases blood pressure
or adults o 18 years or older in the arteries. Branches o arteries called arterioles have a particularly
measured in this way are: high density o muscle cells that respond to various hormone and neural
signals to control blood fow to downstream tissues. Vasoconstriction o
systolic 90-119 mmHg arterioles restricts blood fow to the part o the body that they supply and
diastolic 60-79 mmHg the opposite process, called vasodilation, increases it.
Figure 4 Blood pressure monitor
292
The words you are searching are inside this book. To get more targeted content, please make full-text search by clicking here.
Biology - Course Companion - Andrew Allott and David Mindorff - Oxford 2014
Discover the best professional documents and content resources in AnyFlip Document Base.
Search
IB Biology - Course Companion - Oxford 2014
- 1 - 50
- 51 - 100
- 101 - 150
- 151 - 200
- 201 - 250
- 251 - 300
- 301 - 350
- 351 - 400
- 401 - 450
- 451 - 500
- 501 - 550
- 551 - 600
- 601 - 650
- 651 - 700
- 701 - 730
Pages: