abstract:The origin and divergence of the three living orders of amphibians (Anura, Caudata, Gymnophiona) and their main lineages are one of the most hotly debated topics in vertebrate evolution. Here, we present a robust molecular phylogeny based on the nuclear RAG1 gene as well as results from a variety of alternative independent molecular clock calibrations. Our analyses suggest that the origin and early divergence of the three living amphibian orders dates back to the Palaeozoic or early Mesozoic, before the breakup of Pangaea, and soon after the divergence from lobe-finned fishes. The resulting new biogeographic scenario, age estimate, and the inferred rapid divergence of the three lissamphibian orders may account for the lack of fossils that represent plausible ancestors or immediate sister taxa of all three orders and the heretofore paradoxical distribution of some amphibian fossil taxa. Furthermore, the ancient and rapid radiation of the three lissamphibian orders likely explains why branch lengths connecting their early nodes are particularly short, thus rendering phylogenetic inference of implicated relationships especially difficult.* E-mail: diegos@mncn.csic.es. † E-mail: vences@science.uva.nl. ‡ E-mail: marina@mncn.csic.es. § E-mail: rafaz@mncn.csic.es. Keywords: amphibian evolution, Pangaea breakup, molecular phylogeny, molecular clock, multiple calibrations, RAG1.Living amphibians (Lissamphibia) are a successful and highly diversified group of vertebrates that includes thousands of forms (5,770 species; AmphibiaWeb, January 26, 2005; http://www.amphibiaweb.org/) distributed throughout most habitats in all continents except Antarctica (Duellman and Trueb 1994). They experienced a long evolutionary history dating back at least to the early Triassic, the earliest known fossils being Triadobatrachus from Madagascar (Rage and Rocek 1989) and Czatkobatrachus from Poland (Evans and Borsuk-Bialynicka 1998). The Lissamphibia are widely thought to be a monophyletic group, constituted by three monophyletic orders (Anura, Caudata, and Gymnophiona) whose origin and interrelationships remain hotly debated (see Meyer and Zardoya 2003 for a recent review). The poor fossil record of some major lissamphibian groups and the fact that the three living amphibian orders possibly acquired their specialized morphology very early in their evolutionary histories (Zardoya and Meyer 2001) have left many questions unresolved regarding the origins, relationships, and historical distribution of the Lissamphibia.A recent molecular phylogeny of lissamphibians based on mitochondrial rRNA genes grouped salamanders and caecilians to the exclusion of frogs and suggested that the early evolutionary history of living amphibians was associated with the Mesozoic continental fragmentation of the supercontinent Pangaea (Feller and Hedges 1998). Paradoxically, some distributional patterns and some data from the fossil record (Estes and Wake 1972;Estes and Reig 1973;Rage and Rocek 1989;Jenkins and Walsh 1993;Duellman and Trueb 1994;Evans et ...

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We used partial sequences of the cytochrome b mitochondrial DNA (mtDNA) gene, obtained from 76 individuals representing 45 populations of Iberian Salamandra salamandra plus 15 sequences of additional species of Salamandra and related genera, to investigate contact zones. These zones, identified by earlier allozymic and morphological analyses, are between populations of viviparous (S. s. bernardezi and S. s. fastuosa) and ovoviviparous (S. s. gallaica and S. s. terrestris) salamanders. The distribution of mtDNA and nuclear markers is mostly concordant at one contact zone (between S. s. gallaica and S. s. bernardezi), but at another (between S. s. fastuosa and S. s. terrestris) the markers are offset by about 250 km. The observed geographic variation fits a model of mtDNA capture. Among the potential mechanisms responsible for such discordance we favor a combination of range shifts due to climatic fluctuations and biased genetic admixture across moving contact zones. We apply our findings to the issue of possible homoplasy in the evolution of viviparity and conclude that viviparity likely arose only once within S. salamandra.

SUMMARYThe way in which novelties that lead to macroevolutionary events originate is a major question in evolutionary biology, and one that can be addressed using the fire salamander (Salamandra salamandra) as a model system. It is exceptional among amphibians in displaying intraspecific diversity of reproductive strategies. In S. salamandra, two distinct modes of reproduction co-occur: the common mode, ovoviviparity (females giving birth to many small larvae), and a phylogenetically derived reproductive strategy, viviparity (females producing only a few large, fully metamorphosed juveniles, which are nourished maternally). We examine the relationship between heterochronic modifications of the ontogeny and the evolution of the new reproductive mode in the fire salamander. The in vitro development of embryos of ovoviviparous and viviparous salamanders from fertilization to metamorphosis is compared, highlighting the key events that distinguish the two modes of reproduction. We identify the heterochronic events that, together with the intrauterine cannibalistic behavior, characterize the derived viviparous reproductive strategy. The ways in which evolutionary novelties can arise by modification of developmental programs can be studied in S. salamandra. Moreover, the variation in reproductive modes and the associated variation of sequences of development occur in neighboring, conspecific populations. Thus, S. salamandra is a unique biological system in which evolutionary developmental research questions can be addressed at the level of populations.

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