Lineage divergence and species formation in plethodontid salamanders

The processes by which populations diverge to become separate species underlie broad-scale patterns of species diversity and distribution. I investigated the timing, mode and geographic patterns of population divergence and species formation in plethodontid salamanders using a combination of phylogeographic, physiological and morphological data. Phylogeographic study of Web-toed Salamanders (genus Hydromantes) in the Sierra Nevada showed evidence for both recent and more ancient species formation, and revealed a disconnect between neutral genetic and morphological divergence. While lineages and geographic groups of Hydromantes differed in morphology, only Hydromantes brunus, a species of recent origin, appears to have changed its physiological limits. Physiological constraints may have played some role in preventing gene flow between two historically isolated lineages of Hydromantes platycephalus, but the wide physiological tolerance of these lineages suggest that factors other than physiology were also involved in maintaining these two lineages in allopatry. Phylogeographic data were used at a wider geographic scale to examine how differences in climatic variability between the temperate zone and the tropics could lead to higher levels of population differentiation and allopatric speciation in the tropics. Consistent with this hypothesis, population differentiation was correlated with climatic distance in a higher proportion of the neotropical salamander species examined compared to those from the temperate zone. While the neotropical salamanders are an excellent system for the study of processes generating species diversity, many of these species were shown to have undergone major declines in abundance. Field surveys and comparison with data from field notes from past decades uncovered a previously unrecognized decline in highland terrestrial microhabitat specialists of neotropical salamanders. The correspondence between microhabitat preference, elevation and degree of decline suggests that climatic change may be responsible for these declines, and differences in physiology and exposure to changing climatic conditions could explain differences in severity of decline between species. Further work is necessary to understand the processes responsible for the destruction of diversity in neotropical salamanders.