Genetic variability maintained by mutation and overdominant selection in finite populations

Area of Science:

• Population Genetics
• Evolutionary Biology
• Molecular Evolution

Background:

• Allozyme polymorphism is a key aspect of genetic diversity within populations.
• Understanding the evolutionary forces maintaining genetic variation is a central challenge in biology.
• Overdominance, where heterozygotes have higher fitness, is one proposed mechanism for polymorphism maintenance.

Purpose of the Study:

• To mathematically analyze the overdominance model incorporating mutation and genetic drift.
• To compare the effects of overdominant selection with neutral mutation on heterozygosity levels.
• To investigate the distribution of heterozygosity and allele frequencies under overdominance.

Main Methods:

• Stochastic differential equations (Itô and McKean) were employed for mathematical analysis.
• The study models the interplay between selection (s), mutation rate (nu), and effective population size (N).
• Analysis focused on heterozygosity distributions, allele frequencies, and their relationship with genetic parameters.

Main Results:

• Overdominant selection powerfully increases mean heterozygosity compared to neutral mutations.
• Low mutation rates suffice for observed allozyme polymorphism when 2Ns > 10.
• Heterozygosity distributions and allele frequencies under overdominance differ markedly from neutral models, often showing peaks at intermediate frequencies.

Conclusions:

• Overdominance is a potent force in maintaining genetic variation, particularly allozyme polymorphism.
• The mathematical framework provides insights into the conditions under which overdominance can explain observed genetic diversity.
• Results challenge neutral theory explanations for certain patterns of genetic polymorphism.