Random events in population genetics - Summary
• In a small population, random sampling of gametes to produce the next generation can change the gene frequency. These random changes are called genetic drift.
• Genetic drift has a larger effect on gene frequencies if the population size is small than if it is large.
• If a small population colonizes a new area, it is likely to carry all the ancestral population's genes; but the gene frequencies may be unrepresentative.
• One gene can be substituted for another by random drift. The rate of neutral substitution is equal to the rate at which neutral mutations arise.
• In a small population, in the absence of mutation, one allele will eventually be fixed at a locus. The population will eventually become homozygous. The Hardy-Weinberg equilibrium does not apply to small populations. The effect of drift is to reduce the amount of variability in the population.
• The amount of neutral genetic variability in a population will be a balance between its loss by drift and its creation by new mutation.
• The 'effective' size of a population, which is the population size assumed in the theory of population genetics for small populations, should be distinguished from the size of a population that an ecologist might measure in nature. Effective population sizes are usually smaller than observed population sizes.