Genetic Change in Populations (College Board AP® Biology): Study Guide

Changing allele frequencies & mutation

• Evolution occurs due to natural selection, but it can also be driven by random occurrences that result in changes to allele frequencies over time

  • Allele frequency = how common an allele is within a population’s gene pool

  • When allele frequencies change between generations, this indicates that evolution is occurring

• Random events that can causes changes in allele frequencies include:

  • mutation

  • genetic drift

  • migration

Mutation

• Mutation is a random process that adds new genetic variation to a population when it generates new alleles

  • Mutation is the only source of variation in asexually reproducing species

• New alleles can provide new phenotypes on which natural selection acts

  • An advantageous allele is more likely to be passed on to the next generation because it increases the chance that an organism will survive and reproduce

  • A disadvantageous allele is less likely to be passed on because an organism with such a mutation is less likely to survive and reproduce

• Mutations in a species are essential for evolution by natural selection

  • Note that a mutation taking place in a body, or somatic, cell will not be passed on to successive generations, and so will have no impact on natural selection

Genetic drift

• Genetic drift is defined as:

a non-selective process that occurs when chance events influence the passing on of alleles to the next generation

• Only half of an individual's alleles are present in the gametes, meaning that some alleles will not be passed on due to the random events of meiosis and fertilization

• Over time some alleles may disappear from a population purely by chance, leading to changes in allele frequencies

• Small populations are more susceptible to genetic drift; this is because random events cause proportionally larger changes in allele frequencies when there are fewer individuals

  • E.g. for allele A that starts with a frequency of 0.5:

    • In a small population of 10 individuals there will be 20 alleles, 10 of which will be allele A; by chance the next generation may easily show a change to 8 or 12 copies of A, so the frequency may quickly change to 0.4 or 0.6

    • In a large population of 1000 there will be 2000 alleles, 1000 of which will be A; because random sampling error is tiny with so many copies, the chance of getting as few as 800 or as many as 1200 copies of A in the next generation is extremely low, so the next generation’s frequency stays close to 0.50

• This means that small populations can diverge genetically from other populations of the same species very easily, even without selective pressures

• This can be seen clearly in:

  • the bottle neck effect

  • the founder effect

The bottle neck effect

• The bottle neck effect occurs when:

a large population undergoes a dramatic reduction in size, for at least one generation, leading to a loss of genetic diversity

• The bottle neck effect can occur due to natural disasters which result in the death of many members of a population

• The survivors of bottleneck events are often closely related, limiting genetic variation, e.g.:

  • cheetahs experienced a bottleneck ~10,000 years ago during drastic climate change

  • the surviving population was isolated and inbreeding occurred, meaning that modern cheetah populations have limited genetic diversity

The founder effect

• The founder effect occurs when:

a small group of individuals separates from a larger population to establish a new one, resulting in reduced genetic diversity

• This can occur when a chance event, like a storm, isolates a few individuals

• There will be a limited number of alleles in this new founder population, so genetic variation will be low and the frequency of genes and traits will shift, e.g.:

  • a small number of lizards with white scales travel to a new island via floating debris, separating them from the main population of white and yellow-scaled lizards

  • these few individuals may establish a population with only a subset of the original genetic variation, such as only having white scales

Migration & gene flow

• Migration is the movement of individuals into or out of a population

• Migration can result in gene flow; the addition or removal of alleles from a population

• Gene flow between two populations prevents them from diverging into separate species

  • When there is gene flow between two populations of the same species, the mixing of their gene pools can counteract the effects of natural selection and genetic drift; this prevents any accumulation of genetic differences between the populations