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Many biodiversity hotspots are located in montane regions, especially in the tropics. A possible explanation for this pattern is that the narrow thermal tolerances of tropical species and greater climatic stratification of tropical mountains create more opportunities for climate-associated parapatric or allopatric speciation in the tropics relative to the temperate zone. However, it is unclear whether a general relationship exists among latitude, climatic zonation and the ecology of speciation. Recent taxon-specific studies obtained different results regarding the role of climate in speciation in tropical versus temperate areas. Here, we quantify overlap in the climatic distributions of 93 pairs of sister species of mammals, birds, amphibians and reptiles restricted to either the New World tropics or to the Northern temperate zone. We show that elevational ranges of tropical- and temperate-zone species do not differ from one another, yet the temperature range experienced by species in the temperate zone is greater than for those in the tropics. Moreover, tropical sister species tend to exhibit greater similarity in their climatic distributions than temperate sister species. This pattern suggests that evolutionary conservatism in the thermal niches of tropical taxa, coupled with the greater thermal zonation of tropical mountains, may result in increased opportunities for allopatric isolation, speciation and the accumulation of species in tropical montane regions. Our study exemplifies the power of combining phylogenetic and spatial datasets of global climatic variation to explore evolutionary (rather than purely ecological) explanations for the high biodiversity of tropical montane regions.

A classic paradigm in evolutionary biology is that geographically isolated clades inhabiting similar selective regimes will diversify to create similar sets of phenotypes in different locations (e.g., similar stickleback species in different lakes, similar Anolis ecomorphs on different islands). Such parallel radiations are not generally expected to occur in sympatry because the available niche space would be filled by whichever clade is diversified first. Here, we document a very different pattern, the parallel evolution of similar body-size morphs in three sympatric clades of plethodontid salamanders (Desmognathus, Plethodon, Spelerpinae) in eastern North America. Using a comprehensive, time-calibrated phylogeny of North American plethodontids from nuclear and mitochondrial DNA sequences, we show that these three clades have undergone replicated patterns of evolution in body size and that this parallel diversification occurred in broad-scale sympatry. At the local scale, we find that coexisting species from these clades are more similar in body size than expected under a null model in which species are randomly assembled into communities. These patterns are particularly surprising in that competition is known to be important in driving phenotypic diversification and limiting local coexistence of similar-sized species within these clades. Although parallel diversification of sympatric clades may seem counterintuitive, we discuss several ecological and evolutionary factors that may allow the phenomenon to occur.

Adaptive radiations have served as model systems for quantifying the build-up of species richness. Few studies have quantified the tempo of diversification in species-rich clades that contain negligible adaptive disparity, making the macroevolutionary consequences of different modes of evolutionary radiation difficult to assess. We use mitochondrial-DNA sequence data and recently developed phylogenetic methodologies to explore the tempo of diversification of eastern North American Plethodon, a species-rich clade of woodland salamanders exhibiting only limited phenotypic disparity. Lineage-through-time analysis reveals a high rate of lineage accumulation, 0.8 species per million years, occurring 11-8 million years ago in the P. glutinosus species group, followed by decreasing rates. This high rate of lineage accumulation is exceptional, comparable to the most rapid of adaptive radiations. In contrast to classic models of adaptive radiation where ecological niche divergence is linked to the origin of species, we propose that phylogenetic niche conservatism contributes to the rapid accumulation of P. glutinosus-group lineages by promoting vicariant isolation and multiplication of species across a spatially and temporally fluctuating environment. These closely related and ecologically similar lineages persist through long-periods of evolutionary time and form strong barriers to the geographic spread of their neighbours, producing a subsequent decline in lineage accumulation. Rapid diversification among lineages exhibiting long-term maintenance of their bioclimatic niche requirements is an under-appreciated phenomenon driving the build-up of species richness.

Contemporary North American drainage basins are composites of formerly isolated drainages, suggesting that fragmentation and fusion of palaeodrainage systems may have been an important factor generating current patterns of genetic and species diversity in stream-associated organisms. Here, we combine traditional molecular-phylogenetic, multiple-regression, nested clade, and molecular-demographic analyses to investigate the relationship between phylogeographic variation and the hydrogeological history of eastern North American drainage basins in semiaquatic plethodontid salamanders of the Eurycea bislineata species complex. Four hundred forty-two sequences representing 1108 aligned bases from the mitochondrial genome are reported for the five formally recognized species of the E. bislineata complex and three outgroup taxa. Within the in-group, 270 haplotypes are recovered from 144 sampling locations. Geographic patterns of mtDNA-haplotype coalescence identify 13 putatively independent population-level lineages, suggesting that the current taxonomy of the group underestimates species-level diversity. Spatial and temporal patterns of phylogeographic divergence are strongly associated with historical rather than modern drainage connections, indicating that shifts in major drainage patterns played a pivotal role in the allopatric fragmentation of populations and build-up of lineage diversity in these stream-associated salamanders. More generally, our molecular genetic results corroborate geological and faunistic evidence suggesting that palaeodrainage connections altered by glacial advances and headwater erosion occurring between the mid-Miocene and Pleistocene epochs explain regional patterns of biodiversity in eastern North American streams.