A Test of the Snowball Theory for the Rate of Evolution of Hybrid Incompatibilities

Rolling Snowballs The genetic incompatibilities that separate ongoing speciation events have been hypothesized by the Dobzhansky-Muller model of speciation to snowball—that is, accumulate mutations causing postzygotic isolation at a faster rate than the linear accumulation of mutations. This occurs because of potential deleterious epistatic interactions in hybrids involving two or more interacting genes. Testing QTLs (quantitative trait loci) in seed and pollen sterility between multiple species pairs in the plant group Solanum , Moyle and Nakazato (p. 1521 ) show that hybrid female (seed) sterility accumulates exponentially between increasingly distant species pairs, although not for hybrid male (pollen) sterility. In contrast, loci contributing to differences in other traits show no evidence for nonlinear accumulation over time. Matute et al. (p. 1518 ) come to similar conclusions through the use of deletion mapping in comparisons between two pairs of species of Drosophila . The number of genes causing postzygotic isolation grows as fast as the square of the number of substitutions between two species. Thus, a hybrid snowball effect is found in both plants and animals.