genetic drift & gene flow

Genetic drift

• any change in the allele frequencies of a population that is due to chance

• Random with respect to fitness

• Allele frequency changes are not adaptive

• In a large population, random calamities may not have an effect on allele frequency

• In a small population, genetic drift can be much more pronounced

  • Genetic drift can lead to random loss or fixation of alleles

  • “Allele A is fixed in the population” = the frequency of allele A has reached 100%

Founder effect

• A type of genetic drift (change by chance)

• Genetic drift will occur when a subset of the original population immigrate to a new area and establish a new population

• If the new population is small, the allele frequencies will likely be different from the ancestral population

Genetic bottlenecks

population bottleneck: a sudden restriction in population size

• The probability of all alleles from the population surviving the event is low

• Random with respect to fitness: a random, small selection of alleles survive

Genetic drift is not the same as natural selection after selection pressure event

• ex. In antibiotics, selection pressure is strong

• Only bacteria that contain the mutations that confer penicillin resistance survive

• Drift after population bottleneck → random subset of alleles survive

Gene flow

• occurs when individuals from one population migrate to another population, and the allele frequencies were different in the original populations

• Flow of alleles from one population to another

• Movement of alleles from one population to another

• Tends to equalize allele frequencies among populations

• Microbes: requires genetic recombination or horizontal transfer to be possible

• Prevent divergence rather than enforcing it