HISTORICAL ALLOPATRY AND THE BIOGEOGRAPHY OF SPECIATION IN THE PROSOBRANCH SNAIL GENUS NUCELLA.

Two recently diverged northeastern Pacific sibling snail species, Nucella ostrina and N. emarginata, currently inhabit adjacent zoogeographic provinces. Their distributions overlap in central California to the north of a major faunal boundary at Point Conception, California (PC). To test the hypothesis that modern sympatry is due to a recent northward range expansion by N. emarginata, I analyzed the population structures of both species with nuclear (allozyme) and mitochondrial DNA (mtDNA) markers. Populations of N. emarginata in the region of overlap exhibit significantly lower heterozygosity and allelic diversity than either populations to the south of PC or populations of N. ostrina. A single mtDNA haplotype characterizes all but one population of N. emarginata sampled in this region, but no haplotype to the south of PC is found at more than one locality. MtDNA haplotypes and allozyme allele frequencies also indicate monophyly of central California populations of N. emarginata. Sharp differences in allelic diversity over small geographic distances may reflect the action of natural selection, but because both nuclear and mtDNA markers display concordant patterns, a range expansion across PC best explains patterns of genetic variation in N. emarginata. Allozymes and mtDNA also reveal that the geologically older N. ostrina is paraphyletic with respect to N. emarginata. This pattern is consistent with, but not indicative of, a peripheral isolation model of speciation. Low genetic diversity is also expected if a significant bottleneck occurred at speciation. However, low allelic diversity is not universal throughout the geographic range of N. emarginata; high allelic diversity at the southern end of the distribution of N. emarginata suggests that in the past N. emarginata has been geographically restricted much further south than PC. A northward range expansion across PC by N. emarginata may thus represent only the most recent postglacial movement by the species. The thermal and oceanographic discontinuities found at PC may not have been directly involved in geographic isolation if N. emarginata originated much further south of this modern boundary. Despite uncertainty regarding the exact spatial distribution of populations at speciation, genetic data indicate that even though N. ostrina and N. emarginata currently exhibit a broad range of geographic overlap, speciation was likely allopatric and was initiated by physical isolation of populations in different zoogeographic provinces.