Assortative mating is a form of non-random mating that significantly influences the genetic structure of a species. While random mating, known as panmixia, assumes that all potential partners are equally likely to pair, assortative mating describes a scenario where individuals choose partners based on specific characteristics. This selective pairing acts as a powerful evolutionary driver, creating patterns in the distribution of traits that would not exist under random chance. This mechanism helps shape the genetic diversity and the eventual evolutionary trajectory of a population.
Defining Assortative Mating
Assortative mating is defined as a pattern where individuals with similar or dissimilar phenotypes mate with one another more often than would be expected if mating were purely random. A phenotype refers to an organism’s observable characteristics, which are the result of both its genetic makeup and environmental influences. This selection process is based on measurable traits, such as body size, coloration, specific behaviors, or the timing of reproductive events.
The selection of a mate can be driven by direct preference or by factors like geographical proximity and intrasexual competition. For instance, individuals with similar competitive abilities might occupy the same preferred territories, leading to mating by proximity. Regardless of the underlying cause, the result is a non-random correlation between the traits of the male and female in a mated pair.
Positive and Negative Forms
Assortative mating is categorized into two forms based on the mating preference. The most common form is positive assortative mating, often termed homogamy, which is characterized by “like seeking like.” In this pattern, individuals with a particular trait value show a preference for partners who exhibit a similar trait value.
For example, a taller individual preferentially mating with another taller individual represents positive assortative mating. This mechanism leads to a population where the trait becomes more clustered, as similar traits are repeatedly paired across generations. Conversely, negative assortative mating, also called heterogamy or disassortative mating, occurs when individuals prefer partners who are dissimilar to themselves in a given trait. This pattern is essentially “opposites attract,” such as a large individual mating with a small one, or a preference for complementary traits.
Biological Manifestations
Examples of assortative mating illustrate how this pattern affects diverse species. In three-spined stickleback fish, positive assortative mating exists where individuals of different morphs, or distinct physical forms, tend to mate only within their own group. This preference contributes to the reproductive isolation of the morphs, reducing the chances of hybrid offspring. Positive assortment is also seen in the leaf beetle, where larger males often defeat smaller males in competition to mate with larger females, leading to size-based pairing.
Negative assortative mating is found in the Major Histocompatibility Complex (MHC) of vertebrates, including humans and mice. MHC genes are central to the immune system, and many species use olfactory cues to select mates with dissimilar MHC genes. Mating with an MHC-dissimilar partner increases the genetic diversity of the offspring’s immune system, potentially making them more resistant to a wider range of pathogens. This preference promotes heterozygosity at these specific immune loci, providing a fitness advantage.
Evolutionary Impact
The effect of assortative mating lies in its influence on the distribution of genotypes within a population. Positive assortative mating acts similarly to inbreeding by increasing the proportion of individuals who are homozygous for the trait loci. This means individuals are more likely to possess two identical alleles for the genes controlling the selected trait, which increases the overall genetic variance of the population while decreasing variation within subgroups.
In contrast, negative assortative mating increases the frequency of heterozygotes, meaning individuals are more likely to have two different alleles for the trait. This pattern is a form of balancing selection, which helps maintain higher levels of genetic variation by favoring rare or dissimilar genotypes. Positive assortative mating can be a precursor to speciation, as reduced mating between different trait groups can eventually lead to reproductive isolation and the formation of new species.