Several mechanisms can cause changes in allele frequencies. Primarily these mechanisms are mutation, migration, genetic drift, and natural selection. Natural selection is a mechanism of evolution in which individuals that are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do.
One way to tell if any of these mechanisms are present is to determine whether a population is not evolving. Austrian monk Gregor Mendel came up with rules about the behavior of alleles that apply in this case. He proposed that genes are randomly distributed during sex-cell formation. As such, the expected genotype frequencies for a given combination of allele in a sexually reproducing population that is not evolving can be calculated. A population in which allele and genotype frequencies do not change between generations because evolution is not occurring is said to be in Hardy-Weinberg equilibrium, named after the individuals who proposed the idea.
Realistically, most populations will not stay in Hardy-Weinberg equilibrium for long because to do so is to avoid all possible causes of evolution. Mutation, natural selection, small population size, gene flow, and nonrandom mating can all cause microevolution. Gene flow is a change in the gene pool caused by individuals entering or leaving a population. Nonrandom mating is reproductive pairing in which mates are chosen based on attributes or opportunity, not random selection. With all of the possible combinations of genes, mutation is a natural part of the reproductive cycles of organisms. While not all mutations produce viable offspring, some do. Further, in an environment that is constantly changing, it is likely that natural selection, where some individuals reproduce better than others, will come into play. Additionally, with changes in environments and natural habitats, organisms moving in and out of populations can alter gene flow and population sizes. Nonrandom mating, in which organisms choose their mates, also leads to microevolution as genes from the mates not selected are filtered from the population