Genetic architecture of fitness and nonfitness traits: empirical patterns and development of ideas

Comparative studies of the genetic architecture of different types of trait s were initially prompted by the expectation that traits under strong direc tional selection (fitness traits) should have lower levels of genetic varia bility than those mainly under weak stabilizing selection (nonfitness trait s). Hence, early comparative studies revealing lower heritabilities of fitn ess than nonfitness traits were first framed in terms of giving empirical s upport for this prediction, but subsequent treatments have effectively reve rsed this view. Fitness traits seem to have higher levels of additive genet ic variance than nonfitness traits - an observation that has been explained in terms of the larger number loci influencing fitness as compared to nonf itness traits. This hypothesis about the larger functional architecture of fitness than nonfitness traits is supported by their higher mutational vari ability, which is hard to reconcile without evoking capture of mutational v ariability over many loci. The lower heritabilities of fitness than nonfitn ess traits, despite the higher additive genetic variance of the former, occ ur because of their higher residual variances. Recent comparative studies o f dominance contributions for different types of traits, together with theo retical predictions and a large body of indirect evidence, suggest an impor tant role of dominance variance in determining levels of residual variance for fitness-traits. The role of epistasis should not be discounted either, since a large number of loci increases the potential for epistatic interact ions, and epistasis is strongly implicated in hybrid breakdown.