Abstract
The ABO locus in humans is characterized by elevated heterozygosity and very similar allele frequencies among populations scattered across the globe. Using knowledge of ABO protein function, we generated a simple model of asymmetric negative frequency dependent selection and genetic drift to explain the maintenance of ABO polymorphism and its loss in human populations. In our models, regardless of the strength of selection, models with large effective population sizes result in ABO allele frequencies that closely match those observed in most continental populations. Populations must be moderately small to fall out of equilibrium and lose either the A or B allele (Ne ≤ 50) and much smaller (N e ≤ 25) for the complete loss of diversity, which nearly always involved the fixation of the O allele. A pattern of low heterozygosity at the ABO locus where loss of polymorphism occurs in our model is consistent with small populations, such as Native American populations. This study provides a general evolutionary model to explain the observed global patterns of polymorphism at the ABO locus and the pattern of allele loss in small populations. Moreover, these results inform the range of population sizes associated with the recent human colonization of the Americas.
Highlights
The maintenance of genetic variation has important consequences because heritable genetic variation fuels the evolutionary process
negative frequency dependent selection (NFDS) at the ABO locus should be extremely robust in its ability to maintain variation if it is to explain patterns of polymorphism in most human populations
To verify that our approach correctly modeled the process of genetic drift, we examined the probability that the O allele fixed under complete neutrality (i.e. z = 0.0)
Summary
The maintenance of genetic variation has important consequences because heritable genetic variation fuels the evolutionary process. Balancing selection is of particular interest because it can produce stable genetic polymorphic systems [1]. Balancing selection serves as an umbrella term for several distinct processes (i.e., negative frequency dependent selection, heterozygote advantage, or fluctuating selection) that maintain higher than expected levels of heterozygosity and allelic diversity within populations. Examples of strong balancing selection are compelling, as such a mode of selection can have profound impacts on patterns of genetic diversity across the genome [2]. In flowering plants and fungi, support for balancing selection has been taken from studies of single-locus self-incompatibility systems, whereby individuals.
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