HYBRIDIZATION AND ADAPTIVE EVOLUTION IN PLIO-PLEISTOCENE STROMBID SNAILS FROM FLORIDA

Hybridization between two genetically distinct species may have significant evolutionary consequences through the introduction of phenotypes with competitive and defensive advantages. Of the seven recognized living strombid gastropod species in the western Atlantic, two are thought to interbreed and produce viable hybrid offspring –Strombus alatus, common in the southeastern United States, and its Caribbean congener, Strombus pugilis. We examined the potential for hybridization between these species using a rapidly evolving mitochondrial gene region (cytochrome oxidase I) and morphological traits of the shell from two areas of putative range overlap in Florida. Our preliminary work indicates discordance between morphometric and molecular data with some S. pugilis haplotypes exhibiting S. alatus shell morphology. These results suggest that hybridization is likely occurring between these species.

The fossil record of the S. alatus complex in Florida, which is represented by a single evolving lineage, indicates that a smooth-shelled form dominated throughout much of the Plio-Pleistocene history of the group. Weakly-knobbed forms are rare (0-4%) in the Pliocene, but a strongly-knobbed form, characteristic of living S. alatus, appears in the early Pleistocene and remains at moderately low frequencies (17-28%) until the late Pleistocene. Today the strongly-knobbed form dominates, with nearly 100% of individuals expressing the trait. While selective losses of smooth-shelled forms likely contributed to the evolutionary increase in knobbed individuals (due to the defensive function of the trait against predators), directional selection may only partly explain the adaptive trend. We hypothesize that introgressive hybridization contributed to the increase in occurrence of the strongly-knobbed form of S. alatus in the climatically variable Pleistocene of Florida due to convergence in morphology following secondary contact with strongly-knobbed S. pugilis from the Caribbean.