A Molecular Phylogenetic Perspective on Salamander Evolution

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The purpose of this research is to reveal the evolutionary relationships among the different groups of salamanders by studying the information stored in their genetic material (DNA). Information contained in the separately inherited nuclear DNA and mitochondrial DNA is used to provide two independent inferences of the evolutionary relationships of the different groups of salamander. The evolutionary relationships and historical diversification of a group of organisms constitute essential knowledge for understanding the group's diversity, value as a natural resource and need for conservation. Nine major groups of salamanders are currently recognized as taxonomic families (Ambystomatidae, Amphiumidae, Cryptobranchidae, Dicamptodontidae, Hynobiidae, Plethodontidae, Proteidae, Salamandridae and Sirenidae); earlier results of this project suggest that the Dicamptodontidae as currently recognized incorrectly groups two very different evolutionary lineages. The evolutionary relationships of the salamander families to each other and the relationships of the major lineages contained within each family constitute the focus of this work. The experimental procedure consists of purifying DNA from specimens, isolating comparable ("homologous") segments of DNA, obtaining the nucleotide sequence of those DNA segments, and using statistical tests to discriminate alternative hypothesis of salamander evolution. The patterns of relationship among species are then used to investigate the processes by which the organismal diversity observed in each group arose, giving special attention to developmental patterns. Evolutionary relationships among the salamander families are used to investigate the evolution of larval reproduction, which has occurred multiple times in salamanders. Evolutionary relationships within the Plethodontidae and Salamandridae address the evolution of the diverse, specialized feeding mechanisms that are observed in these families. All levels of this analysis contribute to a revised taxonomic classification of the salamanders. This work also reveals the evolutionary properties of the nuclear and mitochondrial DNA sequences studied. Particular attention is given to evaluating whether the molecular evolutionary pattern meets the assumptions of the analytical methods used to obtain evolutionary information from DNA sequence comparisons. This result provides general information regarding the best analytical methods to use and the magnitude of evolutionary divergence times over which these molecular comparisons will be informative. This facilitates the use of molecular sequence comparisons for conducting similar studies on other groups of organisms.