Negative assortative mating: examples from coat colour in wolves, crown colour in sparrows and <i>HLA</i> in humans

Maintenance of genetic variation which can be maintained by negative assortative mating is important for continued evolutionary response. Coat colour polymorphisms of dominant black coat colour in generally grey wolves is the result of a 3 base pair deletion in the beta-defensin gene. In and near Yellowstone National Park between 1995 and 2024, 59.3% of the matings were observed to be between wolves of different colour, while only 44.9% were expected to be. The dominant white-striped versus tan-striped crown in the white-throated sparrow is associated with inversions greater than 100 Mb in length. Overall, from 1988 to 2019, 98.0% of the observed pairings were between mates with different phenotypes, while only 50% were expected. The human leucocyte antigen (HLA) genes span a large region of about 3.6 megabase pairs on chromosome 6. Studies published in 1995 and 1997 suggested mating choice patterns based on differences in HLA. Since then, there have been a large number of studies that have not found decreased mate sharing of HLA alleles and found mating frequencies consistent with random-mating expectations. Overall, there are only a few well-documented cases of negative assortative mating in vertebrates with the two primary examples being coat colour polymorphism in wolves and crown colour polymorphism in sparrows. However, these examples indicate that negative assortative mating can be very significant and that further efforts to document it are warranted. This article is part of the theme issue 'Exploring negative frequency dependent selection across levels: from genetics to ecology and back again'.