GENETIC COVARIANCES WITHIN AND BETWEEN SPECIES - INDIRECT SELECTION FOR HYBRID INVIABILITY

Genotype-environment interactions and natural selection can result in local specialization when different genotypes are favored in different environments. Restricted gene flow or genetic subdivision enhances lo cal genetic diversification across a species when natural selection ac ts on such variation. The indirect evolution of reproductive isolation and the restriction of gene flow between species in statu nascendi ma y provide a central role for genotype-environment interactions in spec iation genetics. We derive the expected genetic covariance between het erospecific and conspecific viability fitness under several different models of selection, dominance, and breeding structure. Standard quant itative genetic methods can be used to estimate these covariances in e xperimental studies. These genetic covariances permit us to evaluate i n a formal way the indirect effects of selection within a species on t he evolution of hybrid inviability between species. We find that, for autosomal loci and random mating, the genetic covariance across specie s is equal to the product of three quantities: (1) the viability of th e best hybrid genotype; (2) the viability effect of an allele in hybri ds; and, (3) the change in allele frequency due to selection in the co nspecific population. Inbreeding within the conspecific population, ex pressed as Wright's coefficient, F, increases the genetic covariance b y a factor (1 + F). In all cases, a negative genetic covariance across species is evidence for hybrid inviability evolving as an indirect ef fect of selection within species for adaptive (as opposed to neutral) genetic change.