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Facultative or obligate paedomorphosis has evolved several times in Mexican populations of the salamander genus Ambystoma, leading to increased genetic divergence among populations with alternate life histories and contributing to population divergence in this species complex. In the present study, we surveyed the genetic diversity of Ambystoma populations in lakes of the Cuenca Oriental, a high elevation closed drainage basin that encompasses permanent crater lakes harbouring salamander populations. We genotyped individuals from five populations aiming to better understand population dynamics and the evolution of paedomorphosis in this system. Specifically, we tested the hypotheses that the evolution of paedomorphosis in Ambystoma taylori resulted in reduced genetic exchange with populations of Ambystoma velasci in neighbouring lakes. Second, we tested whether the populations in brackish lakes of the Cuenca Oriental, Lake Atexcac, and Lake Alchichica show restricted gene flow across the basin, possibly as a result of local adaptation to those microhabitats. Using various indices of population genetic diversity, Bayesian assignment, and approximate Bayesian computation methods, we show that genetic exchange between brackish lakes and freshwater lakes is negligible, despite continued gene flow among freshwater lakes. We show that the first divergence among populations occurred between Alchichica and the remaining populations and that the evolution of paedomorphosis in A. taylori was likely favoured by local adaptation to saline conditions, thus increasing its genetic isolation. An apparently similar process appears to be in progress independently in lake Atexcac, showing that local adaptation may play an important role in population isolation and, ultimately, in speciation.

The small size and apparent external morphological similarity of the minute salamanders of the genus Thorius have long hindered evolutionary studies of the group. We estimate gene and species trees within the genus using mitochondrial and nuclear DNA from nearly all named and many candidate species and find three main clades. We use this phylogenetic hypothesis to examine patterns of morphological evolution and species coexistence across central and southern Mexico and to test alternative hypotheses of lineage divergence with and without ecomor- phological divergence. Sympatric species differ in body size more than expected after accounting for phylogenetic relationship, and morphological traits show no significant phylogenetic signal. Sympatric species tend to differ in a combination of body size, presence or absence of maxillary teeth, and relative limb or tail length, even when they are close relatives. Sister species of Thorius tend to occupy climatically similar environments, which suggests that divergence across climatic gradients does not drive species formation in the genus. Rather than being an example of cryptic species formation, Thorius more closely resembles an adaptive radiation, with ecomorphological divergence that is bounded by organism-level constraints. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 622-643. ADDITIONAL KEYWORDS: biogeography - diversification - morphology - phylogeny - species formation - systematics.

The largest genus of salamanders, Bolitoglossa (Plethodontidae), is widespread in tropical America, where it occurs in diverse habitats and elevations, from high elevation grasslands to lowland rain forest. It has the most extensive geographical range of any salamander genus. While most species occur in Middle America, it ranges throughout most of tropical South America as well. Phylogenetic analysis of 1196 bp of two mitochondrial genes (cytochrome b, 16S RNA) from 55 species offers strong support for the monophyly of the genus and sorts the species into a number of clades. Taking into account morphology, distribution, general ecology, and prior systematic and taxonomic studies, we recognize seven subgenera, four of them new: Bolitoglossa Dumeril, Bibron et Dumeril, 1854, Eladinea Miranda Ribeiro, 1937, Magnadigita Taylor, 1944, Mayamandra, Nanotriton, Oaxakia and Pachymandra. All South American and some lower Middle American species are included in a single well supported clade, Eladinea. At the species level our analyses uncover the existence of large genetic diversity within morphologically homogeneous taxa. We propose the new combination: B. (Eladinea) paraensis (Unterstein, 1930) stat. nov., for Brazilian salamanders previously included under B. altamazonica. We evaluate evidence for the multiple colonization of the tropical lowlands by morphologically derived species groups. South America was invaded by members of one clade, Eladinea, which we infer to have dispersed to South America prior to closure of the Panamanian Portal. Despite the relatively long history of salamanders in South America, that continent now accounts for a relatively small proportion of the lineages and species of neotropical salamanders. (C) 2004 The Linnean Society of London