Molecular ecology and evolution of the porpoises

Although quite common in terrestrial environments, population differentiation across contiguous geographical areas due to vicariance processes or geographical barriers are far less frequent in the marine environment. Movements in pelagic species such as cetaceans are potentially unrestricted over vast distances. This raises the question of how populations become genetically differentiated, what factors drive individual dispersal, and how speciation occurs in marine pelagic species. The present thesis aimed to investigate how these processes occur at two different evolutionary scales: within (micro-evolution) and between (macro-evolution) species. Combining phylogenetic, phylogeographic, and population genetic approaches complemented with ecological modelling, this thesis investigates the evolution of a group of small cetaceans: the porpoise family (Phocoenidae). The study of the phylogenetic relationships between porpoises as well as their biogeography showed that the seven extant porpoise species radiated during the last 5 million years in response to past environmental changes. At the more recent evolutionary scale within the harbor and finless porpoise species, I investigated the processes partitioning the genetic variation among populations, ecotypes, or subspecies. Analysis of empirical and simulated genetic data shed light on past changes in population size and connectivity, while suitable habitat modelling provided insights into the environmental context in which these demographic events happened. Results revealed that all within species subdivisions arose during the last million years and were strongly influenced by the environmental changes occurring during the Last Glacial Maximum (~26-19 kilo years). The results of this thesis provide a framework for the management and conservation of porpoises.