Origin and phylogeny of Ponto-Caspian crustaceans: Understanding speciation in ancient lakes
This thesis tests hypotheses concerning the evolutionary factors that have influenced the temp of morphological and genetic changes in the crustaceans endemic to the Ponto-Caspian biogeographic region that includes the Black, Azov, Caspian, and Aral Seas. As with other ancient lakes, many groups of aquatic organisms, including crustaceans, molluscs and fishes have radiated in this region. The origin and age of the Ponto-Caspian fauna has been intensively debated, but little is known about the mechanisms of speciation responsible for this high level of endemism. Moreover, the general pattern of morphological stasis that characterizes freshwater crustaceans in many settings does not apply to the Ponto-Caspian cladocerans, amphipods and mysids, which underwent bursts of morphological and ecological diversification. My work investigates the special circumstances that have enhanced morphological reconfiguration in these crustacean groups by comparing the time scale of the molecular divergences within each group with the geological history of the Ponto-Caspian area. The study involves the derivation of molecular phylogenies for groups with contrasting life history strategies, such as the benthic and planktonic crustaceans, using mitochondrial and nuclear gene sequences. The phylogenetic results suggest that young clades such as the Caspian cladocerans provide evidence for sympatric diversification under strong divergent selection, wheras older clades, such as the amphipods and mysids, reveal the role of multiple colonization events and prolonged geographic isolation in taxon radiation. The phylogeographic data establish the presence of genetically distinct assemblages in the Black and Caspian Seas, indicating the role of Pleiocene vicariance events in shaping the evolution of the native fauna. The mechanisms responsible for the high diversity are discussed on the basis of these results. This work also uses phylogeographic data to elucidate the factors responsible for the recent range expansions of Ponto-Caspian crustaceans within Eurpose and North America, their dispersal routes and relative size of the founding populations. The historical patterns of migrations are compared with contemporaneous waves of invasions in order to make inferences about anthropogenic impacts on the distribution of the Ponto-Caspian fauna.