Genetic Consequences of Assortative Mating
Mate selection can have many important evolutionary implications for a population. A randomly mating population implies that any individual with any given genotype has the equal chance of mating with another individual of any given genotype. This is not the case in non-randomly mating populations. In these systems, there exists a greater chance for individuals who mate to either share a common ancestor, such as the case with inbreeding or for individuals to share a similar phenotype, such as the case with assortative mating. Assortative mating, as a form of nonrandom mating, has the potential to act an evolutionary agent. Evolutionary agents violate the Hardy-Weinberg equilibrium and can change both allele and genotype frequencies in a population. The objective of this paper is to assess the impact of assortative mating on population structure and examine the role of assortative mating in evolutionary biology.
Assortative mating is comparable to inbreeding in the sense that mates with similar phenotypes will tend to have similar genotypes. When individuals with similar genotypes mate preferentially to one another this affects genotype frequencies but not gene frequencies. There are two main effects of assortative mating on a population (Crow and Kimura, 1970). First, assortative mating will result in an average increase in homozygosity. This is a result of gametic phase disequilibrium. Homozygosity will cause gametes to become less variable by increasing gametic frequency of homozygote gametes. The second effect of assortative mating is to increase the total population variance. This phenomenon seems counterintuitive since the overall homozygosity of the population is increasing. But for example if we have a trait controlled by 2 loci then at equilibrium the homozygotes that result from purely assortative mating is AABB and aabb, since variance is a deviation these two phenotypes would deviate one from another significantly. Moreover, the only gametes that these two individuals can produce are AB and ab.
Many variables determine the extent to which assortative mating will affect the homozygosity and total variance of a population. Two variables that strongly influence the outcomes of assortative mating are the number of genes involved in creating a phenotype and the degree of assortative mating. For many phenotypes several genes are acting to create a particular trait. As the number of genes involved increases, assortative mating has a much reduced capability to increase homozygosity at any one locus. The degree of assortative mating also influences the genetic effects on the populations. The correlation coefficient r estimates phenotypic likeness of the individuals and thus the degree of assortative mating. If r = 1 then individuals are mating completely assortatively. Under complete assortative mating, phenotypically identical individuals will only mate to one another. Complete assortative mating at one locus will eventually lead to complete homozygosity in the population; otherwise, if r < 1 the population will approach an equilibrium between homozygosity and heterozygosity.
In populations it is difficult to unmask the effects of assortative mating. A population may select assortatively for some traits and not for others. For example, humans mate assortatively with respect to intelligence and height but not blood groups (Crow and Kimura, 1970). Moreover, some traits, like socioeconomic class, may not have a genetic component at all. To further complicate matters, the genes that make up a phenotype can all have different correlation coefficients and degrees of dominance. The effects of assortative mating can not be easily measured when so many variables must be taken into account. Today, scientists develop complex algorithms and models to predict the effects of assortative mating on population structure and equilibrium.
Assortative mating has been observed in variety of species including fruit flies, seahorses, snow petrels and even hermaphroditic nudibranches. In Drosophila subobscura, yellow males tend to mate with yellow females (Rendel, 1944). Female seahorses tend to mate with males seahorses of similar size (Vincent, 1995). Vincent et al. attribute this selection to the high reproductive success observed for similar sized seahorses. The female seahorse will deposit her eggs into the male seashores pouch, which will not only fertilize but carry the eggs until birth.
Snow petrels are birds that make their nests among the rocks on the Antartic mainland. Snow petrels lay only one egg for the duration of their 92 day breeding cycle for which 50 days the egg must be incubated. Two subspecies of snow petrels, Pagodroma nivea major and Pagodroma nivea minor, exist which are differentiated only by size. In areas where the two subspecies cohabitate, researchers observed mixed mating between the sizes, high mate fidelity and even divorce (Jouventin and Bried, 2000). Nest quality is very important to the snow petrel because in a bad year over 40% on the nests can be obstructed by ice therefore snow petrels choose their mate based on nest site and overlook size differences. Moreover, snow petrels are unlikely to leave a mate who has a quality nest thus increasing their reproductive success.
Even ocean dwelling hermaphroditic invertebrates appear to show evidence of assortative mating by size. Nudibraches, commonly called sea slugs, are on average less than two inches long and despite being hermaphrodites these creatures must still mate with another nudibranch. In laboratory experiments, researchers observed a high degree of assortative mating between the sizes of mating pairs (Crozier, 1917). The importance of mate selection is a behavior that appears to transcend species and therefore must have an important evolutionary role.
Assortative mating acting on a gene or set of genes can not change allelic frequency, however, assortative mating can enhance and speed up the change of allelic frequency through natural selection. Assortative mating by increasing total variation in the population can be advantageous to adaptation and species survival. Population structure can transition from one phenotypic state to a better adapted state through assortative mating. If a new combination of alleles is phenotypically better, then assortative mating will prevent this allelic combination from being separated (Williams and Sarkar, 1994). Gametes containing the advantageous allelic combination would increase in the population thus after several hundred or more generations the better adapted phenotype would have a high proportion in the population and represent an overall increase in the mean population fitness (Williams and Sarkar, 1994). Assortative mating and selection can therefore play a role in changing allelic frequencies.
Assortative mating may play a role in sympatric speciation. Speciation occurs when a common gene pool is separated into two allowing for each individual pool to evolve new allele and gene frequencies. Usually, physical or geographical barriers divide a common gene pool, however, sympatric speciation involves the division of a gene pool that is not physically separated. Sympatric speciation is essentially the creation of two species from one species in the same environment. Sympatric speciation is favored when both assortative mating and disruptive selection operate on the same phenotypic character (Rice, 1987). Recently, data was published suggesting seahorses evolved via sympatric speciation. Australian researchers studied a population of seahorses, Hippocampus subelongatus, in the field to observe assortative mating and measure seahorse size (Jones et al, 2003). They also took embryo samples from the male seahorses for microsatellite analysis and genotyped the father and embryo. Maternal alleles were deduced from the known paternal allele and embryo genotype. A model was developed to simulate what would happen in mating seahorse populations of different sizes with different levels of heritability of size. The model predicted if speciation would occur and the mean time it would take. They model suggested that with weak disruptive selection and strong assortative mating, sympatric speciation would occur within hundreds of generations. However, with both strong assortative mating and strong disruptive selection sympatric speciation would occur within tens of generation. The data is only an indirect estimate of sympatric speciation in the evolution of seahorses; however, genetic models are currently the best methods for studying an evolutionary event that may take tens to thousands of years to manifest.
Assortative mating can be as powerful mechanism for genetic change in a population; however the genetic change may take thousands of generations and millions of years to actually be quantifiable. Moreover, because phenotypes are often determined by the interaction of several genes and the environment, predicting the effect of assortative on populations becomes very complicated. In simple cases, where phenotype can be attributed to the interaction at 2 loci with no dominance, it is easier to see that complete assortative mating results in an overall increase in homozygosity and total population variance after infinitely many generations, thus affecting genotype frequencies. However, assortative mating can lead to a change in allele frequency if selection is also acting on the same trait.
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