CONDITIONS FOR SOFT SELECTION FAVORING THE EVOLUTION OF HYBRID INVIABILITY

Hybrid inviability per se is not generally considered adaptive; it is believed to evolve as a pleiotropic consequence of other genetic chang es, either adaptive or neutral, between diverging populations. However , given the pre-existence of hybrid sterility, there are circumstances under which hybrid inviability may be directly selected. In this repo rt, we model the evolution of hybrid inviability in the context of wit hin-family density-dependent selection (soft selection). This paper sh ows that under certain conditions, an allele causing hybrid inviabilit y can increase when rare, despite also decreasing viability in conspec ifics. These conditions depend upon the strength of density effects wi thin families and upon the frequency of matings with heterospecifics. It is found that, under the most favorable conditions (strong soft sel ection and frequent hybridization), the inviability allele can invade when its deleterious effect on the hybrid viability exceeds four times its deleterious effects on conspecific heterozygotes. The conditions for fixation of the allele are also investigated and the circumstances under which this process might operate in natural populations are dis cussed. The conclusion is that the most restrictive condition is that for this process to evolve, there must be little opportunity for postz ygotic reproductive isolation to evolve. (C) 1995 Academic Press Limit ed