| Fluctuating climate over the last 2 million years (MY) has repeatedly caused latitudinal shifts in species distributions, fueling the hypothesis that the glacial-interglacial dynamics of the Pleistocene could have driven regional genetic differentiation and potentially speciation. For species whose distributions spanned the entire North Pacific, regional extinction of northern populations during cooler glacial periods may have resulted in isolation and genetic differentiation of eastern and western populations. To test this hypothesis, I gathered genetic data from a rocky shore intertidal gastropod, Nucella lima, whose current (i.e. warm interglacial) distribution spans the entire North Pacific. Mitochondrial DNA sequences are genetically structured with respect to eastern and western populations, suggesting an extended period of geographic isolation. Two additional nuclear genes also show greater differentiation across the Pacific than among populations on the same side. The population structure of these genes and the amount of genetic divergence and differentiation between the east and west indicate that N. lima persisted in refugia on each side of the Pacific Ocean during the Pleistocene. Indeed, multiple refugia may have existed in the western Pacific each with independent demographic histories subsequent to the last glacial maximum (LGM) due to limited genetic connectivity. Using the isolation-with-migration coalescent model, I have estimated divergence times between eastern and western populations that date within the early Pleistocene and shown that these sampled populations have experienced limited or absent gene flow since the LGM. |
