Genetic admixture

Genetic admixture occurs when individuals from two or more previously separated populations begin interbreeding. Admixture results in the introduction of new genetic lineages into a population. It has been known to slow local adaptation by introducing foreign, unadapted genotypes (known as gene swamping). It also prevents speciation by homogenizing populations.

Genetic admixture often occurs when a geographic barrier separating populations, such as a river or isthmus, is removed or when anthropogenic activities result in movement of populations (for example invasive species).

One example of genetic admixture resulting from the introduction of an invasive species is provided by the Cuban brown anole. Several isolated populations of this species exist in the native range of Cuba. However in the introduced range of Florida, these populations freely interbreed, forming an admixed population.

Another example of a genetic admixture involves a sudden collapse of a natural barrier leading to hybridizations between closely related rival species, such as the grey wolves and the coyotes from the northeastern to the Atlantic regions of North America as well as some parts of the southern US. While wolves and coyotes are closely related and both share a common ancestry, they do not normally interbreed due to the natural hostility between the two species that are known to view each other as competitors. However, between 600 – 900 years ago in the eastern Canada, possible human impacts and persecutions resulting with the decline of the grey wolf populations led to the remnants seeking potential mates in a pre-Columbian coyote population that migrated into the east. The modern day coywolves native to the eastern Canada and the northeastern regions of the US are descendants from the hybrids originating in this genetic admixture.

Mapping
Admixture mapping is a method of gene mapping that makes use of a population of mixed ancestry (an admixed population) to find the genetic loci that contribute to differences in diseases or other phenotypes found between the different ancestral populations. The method is best applied to populations with recent admixture from two populations that were previously genetically isolated for tens of thousands of years, such as African Americans (admixture of African and European populations). The method attempts to correlate the degree of ancestry near a genetic locus with the phenotype or disease of interest. Genetic markers which differ in frequency between the ancestral populations are needed across the genome.

Admixture mapping is based on the assumption that differences in disease rates or phenotypes are due in part to differences in the frequencies of disease-causing or phenotype-causing genetic variants between populations. In an admixed population, these causal variants will occur more frequently on chromosomal segments inherited from one or another ancestral population. The first admixture scans were published in 2005 and since then genetic contributors to a variety of disease and trait differences have been mapped. These include hypertension, multiple sclerosis, BMI, and prostate cancer in African Americans. By 2010, high-density mapping panels had been constructed for African Americans, Latino/Hispanics, and Uyghurs.