Genetic variance is a measure of the variation that exists in the genetic makeup of individuals within population.
Genetic drift is the change in allele frequencies of a population due to random chance events, such as natural disasters.
Gene Flow and Mutation
The gene structure within a population can change as individuals migrate into or out of a population.
Nonrandom Mating and Environmental Variance
Population structure can be altered by nonrandom mating (the preference of certain individuals for mates) as well as the environment.