The percentage of different alleles that exist in a population for a given gene. Allele frequencies don't always match up with phenotype frequencies, as the phenotypes of recessive alleles won't be expressed in heterozygous individuals.

Greater impact upon a smaller population, rather than a large population. When a large populations mating patterns remain random, the allele frequency remains constant. But when a population is small or becoming smaller, allele frequencies will change between generations by chance so begin to drift into an increase or decrease

The changes in allele frequency will be greater in smaller populations, because an individual's alleles represent a greater percentage of the alleles of the population.

When a small population (a small group of individuals) colonises a new isolated area, such as an island and the range and the frequency of alleles that are present within this small group is unlikely to be representative of the original population that the founder population came from.

Occurs when a population suddenly reduces in size due to an extreme event (human or enviromental) that doesn't pick favourable alleles

When there is a rapid decline in the population size, it is likely that the range of alleles will be reduced and the frequency of alleles will change. If the population increases again it will have reduced genetic biodiversity.