We analyzed the rate at which postzygotic incompatibilities accumulate in birds. Our purposes were to assess the role of intrinsic F1 hybrid infertility and inviability in the speciation process, and to compare rates of loss of fertility and viability between the sexes. Among our sample more than half the crosses between species in the same genus produce fertile hybrids. Complete loss of F1 hybrid fertility takes on the order of millions of years. Loss of F1 hybrid viability occurs over longer timescales than fertility: some viable hybrids have been produced between taxa that appear to have been separated for more than 55 my. There is strong support for Haldane's rule, with very few examples where the male has lower fitness than the female. However, in contrast to Drosophila, fertility of the homogametic sex in the F1 appears to be lost before viability of the heterogametic sex in the F1. We conclude that the time span of loss of intrinsic hybrid fertility and viability is often, but not always, longer than the time to speciation. Premating isolation is an important mechanism maintaining reproductive isolation in birds. In addition, other factors causing postzygotic reproductive isolation such as ecological causes of hybrid unfitness, reduced mating success of hybrids, and genetic incompatibilities in the F2s and backcrosses may often be involved in the speciation process.
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Vol. 56 • No. 10