This document discusses factors that affect genetic variation and change in populations, including evolution, natural selection, mutations, migration, and genetic drift. It provides details on each factor and how they influence allele frequencies in a gene pool over multiple generations, leading to evolution and potentially new species. Examples are given to illustrate concepts like founder effects and bottleneck effects on small populations.

1. 22/10/2012
GENETIC VARIATION AND
CHANGE
Biology 2.5 (external: 4 credits)

2. Factors affecting allele frequency
in a gene pool
1. Evolution
2. Natural selection
3. Mutations
4. Migration
5. Genetic drift
a. Founder effect
b. Bottleneck
effect.

3. Genetic Change:
 Gene pool is the total number of alleles present in a
population.
 Genetic change is the change in frequency of alleles
in the gene pool of a population.
 The processes of mutation, natural
selection, migration and genetic drift all affect
the gene pool and change the frequency of the
alleles in that gene pool.
 Frequency of an allele = occurrence of allele _
total number of alleles

4. 1. Evolution:
 Is the process by which new species of
organisms develop from earlier forms.
 Process normally occurs slowly.
 Most often in response to a change in a
species’ environment.
 It is changes in the frequency of the alleles in
a population (some alleles ‘do better’ than
others).
 Evolution acts on populations (i.e. it is
populations that evolve, not individuals).

5. 2. Natural Selection
 The theory of natural selection was proposed by
Charles Darwin over 150 years ago.
 Populations typically produce more offspring than
environmental resources can maintain – there is a
competition for survival.
 Individuals with the best adaptations survive and
reproduce (this is what is meant by fitness) and
pass their successful alleles onto their offspring.
 The frequency of these successful alleles will then
increase in the gene pool.

6.  Environmental factors (both biotic and abiotic)
act as selecting agents of phenotypes.
 When environmental factors change, different
phenotypes will be selected for.
 As phenotype is largely determined by
genotype, successful genotype alleles will
increase in frequency in the gene pool.

7.  Favourable alleles increase in frequency in a
gene pool, while unfavourable alleles decrease.
 If the frequency of alleles changes, evolution is
occurring.
 After a certain number of generations, the
frequency of alleles and phenotypes might
change so markedly that the population becomes
reproductively isolated from others of that
species.
 It is now a new species.

8. DD = warm tolerant
Dd = warm tolerant
Original
dd = cold tolerant
ancestral
population
Cold
environment
Genotypically
isolated gene
pools
Environment
changes Warm
environment
Cold region Further
environmental
changes
Mild region
Selection for different
genotypes as climate
Warm region
changes.

9. 3. Mutations:
 Are the source of new alleles in the gene pool –
mutations are essential for evolution.
 If a mutation occurs in the gamete-producing
cells during DNA replication for meiosis and
that gamete is fertilised, then the mutation will
enter the gene pool.
 Any mutation that enters a gene pool is acted
upon by natural selection.

10.  Alleles resulting from unfavourable mutations are
selected against, and only remain in the gene
pool if they are recessive (remain ‘hidden’ in
heterozygotes).
 Neutral or ‘silent’ mutations (neither favourable
nor unfavourable) are not acted upon by
selection.
 The frequency of these mutated alleles in the
gene pool will be due to chance.
 Eg. a change in a base code (GGG to GGC) that
codes for the same amino acid. The same protein
is made - no change results from this mutation.

11. 4. Migration
 Migration is the movement of individuals from
one population to another.
 Immigration = individuals migrate into a
population.
 Emigration = individuals migrate out of a
population.

12.  Both processes allow for gene flow between
populations.
 Gene flow may change the frequency and/or the
range of alleles in the populations.
 If populations are large, migration may have little
or no effect on allele frequency.
 However, if populations are small, migration may
have a big impact on allele frequency.

13.  Emigration may remove alleles from a
population, reducing a population’s
genetic diversity.
Aa aa
Time
Population after emigrants
have left – frequency of
Emigrants leaving original allele A is greatly reduced
population in the gene pool.

14.  Immigration may add new alleles to a
population, increasing a population’s
Aa aa
genetic diversity.
Original Population Immigrants
Final population after
immigrants arrive – frequency
of allele a is greatly increased
in the gene pool.

15. 5. Genetic Drift
 Is the change in allele frequency due to
change (not selection), and may include
the loss of alleles from the gene pool.
 Most likely to have an effect in small
populations.

16.  When populations are large, mating is
random and the environment is stable (i.e. no
natural selection) – the frequency of alleles
tends to remain stable from generation to
generation.
 However, when populations become
small, allele frequencies can
increase, decrease or even be completely loss
by change alone.
 This has nothing to do with natural selection.

17. a. Founder effect:
 Occurs when a small group of individuals
(founder population) colonises a geographically
isolated area such as an island.
 The range and frequency of alleles in this small
group are unlikely to be representative of those
of the original population – alleles may not be
present or may be more or less frequent.

18.  Likely to be more pronounced in a founder
population.
 Evolution is likely to occur at a faster rate
than in the original or other populations.
 In extreme cases, a founder population
may be a single individual (e.g. a
windblown seed).

19.  Many species arrived in NZ in small numbers by
chance (e.g. birds such as silvereye, white-faced
heron, spur winged plover).
 Or through introduction by humans (e.g.
chamois, tahr).
 They therefore have the potential to become quite
different from the original populations and their
evolution is likely to progress faster as natural
selection by the different environment occurs.

20. b. Bottleneck effect:
 Populations may be suddenly reduced in
numbers.
 Usually from a catastrophic environmental event
(e.g. fire, flood, landslide, or drought).
 Or by sudden, severe selection pressure (often
human activities, e.g. rapid habitat
destruction, introduction of predators/
competitors).
 After the event, the populations may recover to
grow again to return to normal levels.

21.  As population numbers drop rapidly, it is likely
that the range of alleles decreases and the
frequency of alleles changes.
 When small, the population is more subject to
genetic drift.
 When the population increases, it is likely that it
will have reduced genetic diversity.