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

Mutations & genetic variation

• Evolution by natural selection results in changes to allele frequencies over time

• It is also possible for allele frequencies to change as a result of random chance, e.g. due to

  • mutations

  • genetic drift

  • migration

Mutations

• The original source of genetic variation is mutation

• Mutation results in the generation of new alleles which can influence the evolution of a species

• Mutations that take place in the dividing cells of the sex organs lead to changes in the alleles of the gametes that are passed on to the next generation

  • A new allele may be advantageous, disadvantageous or have no apparent effect

  • 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 mutation is more likely to die out 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

• A mutation is the only source of variation in asexually reproducing species

Genetic drift & genetic variation

• Genetic drift is defined as:

a non-selective process that occurs when chance events influence which alleles are passed to the next generation, especially in small populations

• The mechanism of genetic drift is as follows:

  • 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 random fertilization

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

• Genetic drift reduces genetic variation within populations and increases differences between populations

• Small populations are more susceptible to genetic drift; small populations may arise due to:

  • the founder effect

  • population bottlenecks

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

• A chance event, like a storm, isolates a few individuals

• The new population only has the alleles of the founders in its gene pool, making it more vulnerable to genetic drift

  • 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

Population bottlenecks

• The bottleneck effect occurs when:

a large population undergoes a dramatic reduction in size, leading to a loss of genetic diversity

• Events like natural disasters reduce population size

• 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

Migration & genetic variation

• Gene flow, or migration refers to any movement of genetic material or individuals between populations

• Different populations have different gene pools with a different range of alleles

  • This range of alleles can be altered if individuals from one population breed with individuals from another population

  • This interbreeding allows new alleles, that have arisen through mutation, to be introduced into different populations

  • Mixing gene pools in this way increases genetic variation

  • Migration can also lead to genetic drift