Variation stems from mutation, combination, and independent assortment of alleles for a gene.
According to Hardy-Weinberg equilibrium, allele frequencies remain in equilibrium in a population as long as natural selection does not occur, new mutations do not occur, the population size is large enough to offset random or chance changes, mating is random, and organisms with different alleles do not migrate into or out of the population. These conditions are rarely met, therefore microevolution is what usually happens to populations. CA Science Content Standards Grade 1 2a Different plants and animals inhabit different kinds of environments and have external features that help them thrive in different kinds of places. Grade 2 2a Organisms reproduce offspring of their own kind. The offspring resemble their parents and each other.