Experimentally Testing the Hypothesis That Hybrid Fitness Can Increase Evolutionarily: An Example in Spadefoot Toads.

Hybrids often have reduced fitness because of genetic incompatibilities. If populations contain variation at loci involved in these genetic incompatibilities, then selection can disfavor alleles involved in incompatibilities, thereby reducing their frequency within the parent-lineages' gene pools in the hybridizing population. Such selection can concomitantly increase hybrid fitness in populations over evolutionary time. This hypothesis predicts that hybrid fitness should be higher in populations where species have hybridized for longer periods (or where hybridization is more frequent) versus in populations where species have hybridized for a shorter duration (or where hybridization is less frequent). We tested this prediction in spadefoot toads, Spea bombifrons and Spea multiplicata. Spea bombifrons has expanded its range into that of S. multiplicata; consequently, regions of sympatry differ in the length of time during which hybridization has occurred. We used a "space for time substitution" experiment to contrast hybrid fitness, as measured by development time and body size, in different regions of sympatry. We found that hybrids derived from "Old sympatry" were larger and had faster development than hybrids derived from "New sympatry." Despite limits in our study design, our results suggest that selection can reduce incompatibilities and increase hybrid fitness evolutionarily. We discuss means for best evaluating whether selection can evolutionarily reduce genetic incompatibilities. Doing so is important to better understand ecological and evolutionary processes ranging from speciation to hybridization's role in adaptive evolution and species range expansion.