Assortative mating, also known as non-random mating, is the selection of a mate based on certain phenotypic traits that are similar to the individual.1 As reproducing organisms select mates based on desirable traits they decrease the heterogeneity of a population and violate the Hardy-Weinberg Equilibrium theory.
According to Hardy-Weinberg Equilibrium when completely random mating occurs with no other outside factors impacting that population, then that population will have a consistent percentage of genotypic frequency.This genotypic frequency will remain 25% homogenous dominat (AA), 25% will be homoygous recessive (aa), and 50% will be heterozygous (Aa). 2
However, this equilibrium is very unlikely in the natural world. More likely is that choosing of mates not at random. When a mate selects another mate that is similar to themselves they slowly increase the homogeneity of a population. The percentage of homozygous gentoypes (aa, AA) will increase and be selected for. 2
1.Jiang, Yuexin, Daniel I. Bolnick, and Mark Kirkpatrick. "Assortative Mating in Animals." The American Naturalist 181.6 (2013): E125-138. University of Texas. Web. 17 Mar. 2015. <http://www.sbs.utexas.edu/kirkpatrick_lab/k/Kelsey_files/670160.pdf>
2. O'Neil, Dennis. "Non-random Mating." Modern Theories of Evolution: Non-random Mating. N.p., n.d. Web. 17 Mar. 2015. <http://anthro.palomar.edu/synthetic/synth_8.htm>.