Molecular and morphological evolution of the amphipod radiation of Lake Baikal

The amphipods of Lake Baikal, Russia, are a taxonomically and morphologically diverse group, and have been cited as a classic example of adaptive radiation. An unfortunate consequence of this diversity has been an instability of Baikal amphipod classification, with frequent revisions spurred by differing opinions about the phylogenetic utility of different characters. Classifications have never been based on a rigorous phylogenetic analysis of morphological data and only recently has the phylogeny of Lake Baikal\u27s amphipods been examined using molecular tools. Unfortunately, most of these studies have led to a limited expansion of our understanding of Baikal amphipod systematics. Phylogenetic relationships of selected amphipod species from Lake Baikal were constructed using molecular data from portions of two mitochondrial genes, 16S rDNA and COI, and 121 morphological characters. Additionally, patterns of morphological change and the phylogenetic utility of different characters were examined by tracing the 121 morphological characters upon the most parsimonious 16S phylogeny. While the molecular datasets resulted in similar topologies, the morphological data were not congruent with either molecular dataset. A combined molecular/morphological analysis resulted in a topology that was not very different than the combined molecular phylogeny. When plotted on the most parsimonious 16S tree, the morphological data was very homoplastic, although the level of homoplasy differed among types of morphological characters. Contrary to recent popular opinion, body armour and processes characters were the least homoplastic of all types, while characters that have been considered phylogenetically useful, such as mouthpart characters, were the most homoplastic. The molecular phylogenies obtained in this study lead to a better understanding of the phylogenetic history of Baikal\u27s amphipods. The large, morphologically diverse family Acanthogammaridae is strongly supported as a monophyletic group, and likely to have radiated from a single ancestral species within the lake. Conversely, the fossorial, morphologically uniform family Micruropodidae seem to be composed of two relatively unrelated groups, one of which is closely related to the monotypic Macrohectopidae, the lake\u27s morphologically highly modified pelagic planktivore, suggesting a disjunction between morphological and molecular evolution. Finally, this study suggests that the amphipod fauna of Lake Baikal is not monophyletic; the cosmopolitan Gammarus lacustris may share recent common ancestry with some Baikal taxa