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factors affecting allele frequencies

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GENETIC VARIATION AND CHANGE Biology 2.5 (external: 4 credits) 10/06/2022
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Page 1: factors affecting allele  frequencies

GENETIC VARIATION AND CHANGE

Biology 2.5 (external: 4 credits)

12/04/2023

Page 2: factors affecting allele  frequencies

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.

Page 3: factors affecting allele  frequencies

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

Page 4: factors affecting allele  frequencies

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).

Page 5: factors affecting allele  frequencies

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.

Page 6: factors affecting allele  frequencies

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.

Page 7: factors affecting allele  frequencies

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.

Page 8: factors affecting allele  frequencies

DD = warm tolerantDd = warm tolerantdd = cold tolerant

Original ancestral population

Environment changes

Cold region

Mild region

Warm region

Selection for different genotypes as climate changes.

Further environmental changes

Cold environment

Warm environment

Genotypically isolated gene pools

Page 9: factors affecting allele  frequencies

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.

Page 10: factors affecting allele  frequencies

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.

Page 11: factors affecting allele  frequencies

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.

Page 12: factors affecting allele  frequencies

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.

Page 13: factors affecting allele  frequencies

Emigration may remove alleles from a population, reducing a population’s genetic diversity.

Aa aa

Emigrants leaving original population

Time

Population after emigrants have left – frequency of allele A is greatly reduced in the gene pool.

Page 14: factors affecting allele  frequencies

Immigration may add new alleles to a population, increasing a population’s genetic diversity. Aa aa

ImmigrantsOriginal Population

Final population after immigrants arrive – frequency of allele a is greatly increased in the gene pool.

Page 15: factors affecting allele  frequencies

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.

Page 16: factors affecting allele  frequencies

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.

Page 17: factors affecting allele  frequencies

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.

Page 18: factors affecting allele  frequencies

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).

Page 19: factors affecting allele  frequencies

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.

Page 20: factors affecting allele  frequencies

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.

Page 21: factors affecting allele  frequencies

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.


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