Adaptive Differentiation with Little Genic Change Between Two Native California Minnows

We have studied allelic variation at 24 loci coding for soluble proteins in two presumed species of California minnows-Hesperoleucus symmetricus and Lavinia exilicauda. Our estimates indicate that they differ at one electrophoretically detectable codon substitution for every 20 loci. This small amount of genetic divergence is unusual in interspecies comparisons. Nonetheless, there is evidence that Hesperoleucus and Lavinia are different species. They are considerably different in morphological and ecological attributes, and exhibit strong prezygotic isolating barriers. Their distinctness is largely retained also where they are sympatric. The close genic similarity between Hesperoleucus and Lavinia is due in part to their recent separation from a common ancestor, perhaps during the middle or late Pleistocene. In any case, significant adaptive differentiation leading to speciation has occurred within the context of relatively few genic changes.