<P> Population genetics studies the distribution of genetic differences within populations and how these distributions change over time . Changes in the frequency of an allele in a population are mainly influenced by natural selection, where a given allele provides a selective or reproductive advantage to the organism, as well as other factors such as mutation, genetic drift, genetic draft, artificial selection and migration . </P> <P> Over many generations, the genomes of organisms can change significantly, resulting in evolution . In the process called adaptation, selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment . New species are formed through the process of speciation, often caused by geographical separations that prevent populations from exchanging genes with each other . </P> <P> By comparing the homology between different species' genomes, it is possible to calculate the evolutionary distance between them and when they may have diverged . Genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics . The evolutionary distances between species can be used to form evolutionary trees; these trees represent the common descent and divergence of species over time, although they do not show the transfer of genetic material between unrelated species (known as horizontal gene transfer and most common in bacteria). </P> <P> Although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms . The fact that significant research already existed for a given organism would encourage new researchers to choose it for further study, and so eventually a few model organisms became the basis for most genetics research . Common research topics in model organism genetics include the study of gene regulation and the involvement of genes in development and cancer . </P>

Two species that have been used to study the genetic and molecular controls of meiosis are