Evolutionary Causes of Genetic Variation in Fertility and other Fitness Components

Abstract

In humans, fertility and other fitness-related traits (life history traits) usually have low to moderate heritabilities, suggesting genetic variability for these traits. This variation is often attributed to some form of "balancing" natural selection that actively maintains it. However, results from evolutionary genetics suggest that most genetic variation is due to non-adaptive forces such as mutation. In this chapter, we apply results of evolutionary quantitative genetics to existing data on human fertility and other fitness components. First, we compare heritabilities and genetic variances for human traits to similar measures in two model organisms: Drosophila melanogaster (fruit flies) and Mus musculus (house mice), and we argue that genetic variances are much more informative for such comparisons. General results are that genetic variances and heritabilities are similar in humans and in Drosophila. The mean coefficient of genetic variation in humans is 11%, and in Drosophila 9%. Mean heritability in both species is about 0.2. We also describe both adaptive and non-adaptive mechanisms that can maintain variation within populations, and evaluate evidence relating these mechanisms to human variation. An important implication of this analysis is that only about 12% of phenotypic variation in human fitness traits is likely due to genetic polymorphism that is actively maintained by natural selection. The remainder of phenotypic variance for these traits is either non-genetic, or due to nonadaptive forces such as mutation. We discuss ways in which methods and results of evolutionary quantitative genetics can be applied to studies of human traits.