MS Defense of Sedona Ryan

Stepping stones into the desert: an experimental test of hybridization-facilitated range expansion in spadefoot toads. 

Hybridization between species often results in low-fitness offspring with reduced growth, survival, or fecundity. Such low fitness in hybrids is generally thought to derive from genetic incompatibilities, the likelihood of which increases with increasing genetic distance between the hybridizing species. Consequently, less genetically differentiated species are more likely to exchange genes via hybridization, whereas more genetically differentiated species should be unable to exchange genes. However, under the stepping stone model of hybridization, hybridization with a more recently diverged species may contribute to gene exchange in such a way that it reduces genetic incompatibilities with other, more divergent species. Essentially, hybridization with more genetically similar species can act as a stepping stone to hybridization with more divergent species if the acquisition of genetic variation from the former reduces genetic incompatibilities with the latter. To test this possibility, we conducted a controlled breeding experiment between spadefoot toads: pure allopatric Spea bombifrons and two distinct clades of Spea multiplicata. Our goal was to investigate whether hybridization between the two clades revealed differential consequences of hybridization on hybrid fitness as expected by the “stepping stone” model. As predicted, hybrid offspring sired by the north S. multiplicata clade had greater mass, greater length, and developed more rapidly than hybrid offspring sired by the south S. multiplicata clade. These results suggest that hybridization with northern S. multiplicata may have facilitated later hybridization with southern S. multiplicata. This effect of hybridization might have enabled the range expansion of S. bombifrons into the desert southwest.