CHAPTER 3: VARIATION

CHAPTER 3: VARIATION PDT SEMESTER II:DB024

CHAPTER 3: VARIATION

3.1 INTRODUCTION TO VARIATION

Variation

✓ Variation is a difference in characteristic between individuals from the SAME species.
✓ Importance:

▪ Variation increases the ability of a species to survive in a changing environment
▪ Different phenotypes may be able to inhabit a wider range of habitats
▪ Natural selection will increase the frequency of advantageous genes over successive generations
▪ Accumulated genetic differences may lead to new species
▪ A natural population with little variation may reduce its capacity to adapt to the changes in

environmental conditions and this may lead extinction of the population or the whole species
▪ As a source of evolution
✓ There are two types of variation:

i. Continuous (quantitative) variation ii. Discontinuous (qualitative) variation

Variation that shows intermediate phenotypes falls Variation in which no intermediate phenotypes

between the extremes. fall between the extremes.

E.g.: Height E.g.: Garden peas seed shape (wrinkle or smooth),
▪ Weight ▪ Ability to roll tongue
▪ Skin colour ▪ Human blood group
▪ ▪ Earlobes
▪

Continuous and discontinuous variations

Differences between both variation:

Continuous variation Discontinuous variation

Have intermediate phenotypes fall between the No intermediate phenotypes fall between the
extremes// Individuals CANNOT be grouped into extremes // Individuals CAN be grouped into
distinct phenotypic classes distinct phenotypic classes

Influenced by genetic and environment factors Influenced by genetic factor only

Characteristics controlled by many genes Characteristics controlled by one gene

Shows a normal distribution or bell-shaped curve Show discrete distribution curve

Quantitative // Characteristics can be measured Qualitative // Characteristics cannot be measured

Similarities between both variation:

1. Both are controlled by genetic factors

2. Both occur between individuals from the same species

3. May occur during meiosis

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3.2 SOURCES OF VARIATIONS

a) Factors which cause genetic variation ii. Mutation
i. Genetic recombination (i) GENE MUTATION: change in

(i) CROSSING OVER during Prophase I nucleotide base sequence of a gene
- Exchange of genetic materials between non-
sister chromatids of homologous (ii) CHROMOSOMAL MUTATION:
- chromosomes form variability in gametes change in the chromosome number or
structure
(ii) INDEPENDENT ASSORMENT during
metaphase I - Changes in genotype may be inherited if
- Random arrangement of homologous mutations occur in gamete cells
chromosomes on metaphase plate

(iii) RANDOM FUSION OF GAMETES

- Each female gamete can fuse with any of the

male gametes

b) Environmental variation
✓ Causes continuous variation
✓ Cannot be inherited to the next generation
✓ Act to produce phenotypic differences between individuals.
✓ For example, it is due to:

- food intake/ nutrition

- climate (humidity, temperature, light intensity)

- culture (Sikhs grow their hair very long and, but Buddhist monks shave their heads)

Interaction between genetics and environmental factors

Variation also may be due to interaction between genetics and environmental influences

3.3 SELECTION

Selection
▪ Selection is a process of selecting or choosing individuals with certain characteristics to live by

human or nature.
▪ Those selected able to survive, live longer and reproduce. Their genes passed to next generation and

represented more other than unfavorable traits.
▪ Variation can cause selection

Types of selection

i. Natural selection ii. Artificial selection

▪ A process of selection involving environmental ▪ A process of selection involving human

factor as the selecting agent intervention as the selecting agent.
✓ In natural selection, the selection agent is ✓ In artificial selection, the selection agent is

environment. human.
✓ Natural selection is a random event. ✓ Artificial selection is a planned event.
✓ In natural selection, members of a population ✓ It is carried out by selective breeding on

that are more successfully adapted to the organisms showing desired characteristics/

environment have greater fitness. traits.
✓ Natural selection results in favourable ✓ Artificial selection producing a new breeds/

alleles are passed on to the next generation varieties of organisms for a specific purpose.
and less favourable alleles become ✓ There are TWO types of artificial selection:

eliminated. inbreeding and outbreeding.

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✓ There are THREE types of natural selection:

stabilizing selection, directional selection

and disruptive selection. ▪ Example:
▪ Example:

1. Predators eats the more easily seen beetles 1. Selective breeding of dog/ cats //

resulting increases the other gene domesticated dogs

frequency.

TYPES OF SELECTION Types of Natural selection

- - - - - - original population (a) Stabilizing selection
______ new population ▪ Favour intermediate phenotypes by acting against

both extreme phenotypes
✓ Intermediate phenotypes have greater survival &

reproduce successfully
✓ Extreme phenotypes of the population have lesser

chance to survive
▪ Reduces variation because its maintains a particular

phenotypic character
▪ Example: Human baby birth weight

✓ Most human babies with intermediate weight

and size are most likely to survive
✓ Babies who are either much smaller (prone to

illness) or much larger (difficulties during birth)

suffer higher rates of mortality.

(b) Directional selection
▪ Favour one extreme phenotype by acting against the

other extreme phenotypes.
▪ Selection shifts the towards favoured phenotypes

because this phenotype is the fittest with environment

and survive
▪ Common when a population’s environment changes

or when members of a population migrate to a new

habitat
▪ Example 1: Industrial melanism of Biston betularia

(black peppered moth)
✓ Before industrial revolution, population of

light peppered moth are dominant because

they can camouflage with the trees trunk

which are light and covered by lichens.
✓ After industrial revolution, trees trunk was

dark causing the dark peppered moth are

dominant because they can camouflage with

the trees trunk which are darker now.

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(c) Disruptive selection
▪ Favour both extremes phenotype in a population by

acting against intermediate phenotypes.
▪ Two extreme phenotypes are separated and two

subpopulations/ new species will be formed
▪ Example: Variety of size and shapes of beak of

Galapagos finches
✓ Big-size beak birds can feed on the big and hard

seeds while small-size beak birds feed on fine

and soft seeds.
✓ Medium-size beak birds have difficulty eating

big and hard seeds and also have difficulty

finding soft and fine seeds

Types of Artificial selection

(a) Inbreeding (b) Outbreeding

▪ Involves mating between closely related ▪ Involves mating between unrelated or distantly related

individuals individuals of a species or different species.

✓ Use individuals within a population or ✓ Between individuals of different varieties/strain

one subspecies ▪ Leads to an increase in heterozygosity (shows more

✓ E.g.: Self-fertilization in plants variation within population)

▪ Disadvantages: Leads to an increase in ▪ Increases number of heterozygous individual/ hybrid

homozygosity (show less variation within vigour/ heterosis

populations) ✓ Heterosis has a better chance of survival

▪ It leads to inbreeding depression ✓ Increase resistant to diseases, longer lifespan, more

✓ Lower the population’s ability to survive fertile, increased in size & strength

and reproduce ▪ Eg:

✓ Inbred individuals are more susceptible ✓ Cow (Hereford and Aberdeen Angus are bred to

to diseases, shorter lifespan, reduction of produce Angus (hybrid) with high quality and

immune system function, less fertile quantity of meat)
▪ Eexample in domesticated animals & crops: ✓ Cows (Jersey and Guernsey are bred to yield high

✓ Breeding of ‘show’ animals such as cats quantity of milk)
✓ Sheep (Hampshire and Suffolk are bred to produce
& dogs/ domesticated animal for fashion
✓ In agriculture, to produce selected crop high quality of meat)

plants such as wheat and rice

DIFFERENCES BETWEEN INBREEDING AND OUTBREEDING

INBREEDING OUTBREEDING

Mating between closely related individuals Mating between unrelated or distantly related individuals

of a species

Increases homozygosity Increases heterozygosity

Shows less variation Shows more variation

Increases inbreeding depression Increases hybrid vigour/ heterosis

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3.4 SPECIATION

SPECIATION
▪ Based on biological species concept, a species define as a population or group of population whose

members has the potential to
✓ Interbreed under natural conditions
✓ Producing viable and fertile offspring and
✓ Do not produce interbreed with members of other species (Reproductively isolated from other

populations)
▪ Speciation is formation of a new species from pre-existing species through evolution process.
▪ Speciation begins when gene flow is reduced

✓ Gene flow - The transfer of alleles from one population to another, usually due to migration
✓ Gene flow eliminates genetic differences in populations
✓ Lack of gene flow makes a species independent

FACTORS INVOLVED IN FORMATION OF NEW SPECIES

i. REPRODUCTIVE ISOLATION
▪ Isolation is a separation process which leads to speciation.
▪ This mechanism acting before or after fertilization.
▪ Isolation that occurs before fertilization. (prevent mating between individuals or fertilization of egg)

called pre-zygotic isolation.
▪ Isolation that occurs after fertilization or after the hybrid zygote is formed called post-zygotic

isolation

ii. GENETIC DRIFT
i. Genetic drift refers to the change of allele frequencies in a population due to chance.
ii. Two types of genetic drift:

1) Founder effect/principle- Occurs in a small population which is isolated/ migrated from the
original
population.

2) Bottleneck effect- Occurs when there is a sudden decrease in the original population size due to
natural disaster

iii. HYBRIDIZATION (ALLOPOLYPLOIDY)
Phenomenon that occurs when members of
two different species interbreed.

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iv. ADAPTIVE RADIATION

▪ Evolution from one
species of animal or
plant into a number of
different species due to
adaptation to different
habitats or niches.

▪ Example: Sizes and
shapes of beak of
Finches on Galapagos
islands.

END OF TOPIC

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