Lineage Divergence And Species Formation In Plethodontid Salamanders PDF Download

Patterns in the distributions of species result from numerous ecological and evolutionary processes, including competitive interactions, evolved physiological tolerances, and the historical environmental fluctuations that have caused ranges to shift, expand, or contract over long time periods. For some groups of species, these processes have resulted in an elaborate diversification of traits. For other groups, however, such as woodland salamanders (genus Plethodon), closely related species may be phenotypically similar or even identical. It is less clear what drives the formation of such species, whether and how they maintain their identities and interact after initial divergence, and how such similar species might have evolved ecologically in subtle ways to differentially utilize the landscape. The Eastern Red-backed Salamander (Plethodon cinereus) is a small, fully terrestrial woodland salamander notable for its wide distribution and high phylogeographic diversity in the eastern United States and southeastern Canada. Most of its current distribution, however, was covered by ice sheets at the end of the Pleistocene, suggesting recent and rapid post-glacial expansion. It is not yet clear how P. cinereus colonized these landscapes, and what ecological characteristics made P. cinereus so successful in its range expansion compared to other species. Some studies comparing narrowly-distributed montane Plethodon to their lowland counterparts have suggested a combination of physiological specialization and competitive superiority of montane species relative to generalist lowland species such as P. cinereus, but it is unknown if this asymmetry applies to sets of lowland species outside of montane systems. In this dissertation, I explore the biogeography of woodland salamanders, with special attention to P. cinereus, at ecological and evolutionary time scales to uncover the processes that shape genetic diversity and species distributions. Chapters 1 and 2 concern the historical biogeography and evolution of P. cinereus, while Chapters 3 and 4 compare the habitat use of P. cinereus to two of its more broadly distributed relatives, the Northern Ravine Salamander (P. electromorphus) and the Southern Ravine Salamander (P. richmondi), to test if they have diverged ecologically, including multiple ecological scales. In Chapter 1, I used next-generation DNA sequencing to explore the phylogeographic structure and demographic history of P. cinereus, using the species as a model for the formation of incipient lineages of Plethodon. I identified several groups that diverged within the Pleistocene, yet I also found numerous and sometimes extensive regions of admixture between groups, suggesting a model of range fragmentation and fusion during incipient species formation. In Chapter 2, I focus on the most recent post-glacial expansion of P. cinereus, using DNA sequencing from range-wide samples to uncover the origins and routes of colonization. Results generally supported a southeastern coastal origin that expanded northward, westward into the Great Lakes region, and then a novel colonization route southward back into unglaciated areas. In Chapter 3, I used ecological niche modeling to test if niche differentiation between P. cinereus, P. electromorphus, and P. richmondi explains their broad distributional patterns. All species had estimated niches that were more different than expected by chance, and historical habitat suitability has fluctuated dramatically in the Holocene epoch, although I did not find strong evidence that climatic or topographic variables differentiated occurrences of P. cinereus and P. electromorphus within their overlapping distribution. Finally, in Chapter 4, I explore microhabitat differentiation between P. cinereus and P. electromorphus where they occur and interact at fine spatial scales. Plethodon cinereus greatly outnumbered P. electromorphus, and P. electromorphus used cooler, wetter microhabitats that were also occupied by P. cinereus, with no evidence for a negative correlation in their occupancies or abundances. Taken together, my work suggests that current distributions, including patterns within and between species, are shaped by historical range expansion and contractions, as well as subtle differences in habitat use at multiple spatial scales.