Next Tuesday I'll tell you a little about my past PhD-project in Helsinki University. Fika included, see you there!
My thesis in nutshell:
Phylogeography of amphi-boreal marine fauna
The Northern Atlantic and Pacific Oceans share a faunal element consisting of pairs of closely related vicariant taxa, known as the amphi-boreal fauna. The inter-oceanic systematic affinities reflect a history of shared ancestry since dispersal through the Bering Strait and across the Arctic basin, which was initiated by the opening of the Bering Strait at the end of the Miocene. In my PhD I examined the dynamics and consequences of the faunal interchange between the Atlantic and Pacific Oceans since that time up to the present. This was done by comparing differences in the mitochondrial gene sequence variation in different taxonomic groups and across the circum-boreal geographical scale, comprising both newly produced sequences and data from literature, from altogether 70 species or species groups (molluscs, crustaceans, echinoderms, polychaetes, fishes and mammals). Two exemplary genera, pelagic fish genus Clupea and boreal-arctic bivalve Hiatella were examined more closely and with additional markers.
The phylogeographical histories of the Pacific–Atlantic amphi-boreal taxa were found to be remarkably variable. A simple vicariant history since an early, Pliocene or Early Pleistocene (6–3 My ago) dispersal was inferred only in about half of the examined taxa, whereas signatures of more than one trans-Arctic dispersal were found in a third of the cases. A close inter-oceanic relationship that would reflect recent trans-Arctic dispersal or ongoing gene flow was found to be common for amphi-boreal genera (40% of cases). Overall the estimates of inter-oceanic mitochondrial divergence within each of the broader taxonomic groups varied greatly, up to 10–20 fold, and suggest that trans-Arctic faunal dispersal has been a repeated process through the time frame considered. Based on the molecular divergence, several instances of putative new allopatric species were detected in the invertebrates.
Repeated trans-Arctic invasions have practically always resulted in secondary contacts of diverged lineages (for this data, in all cases but one). The Pacific herring C. pallasii, of East Asian origin, invaded the Northeast European seas post-glacially and then differentiated into separate regional populations. In Europe, hybridization with the native sister species C. harengus, the Atlantic herring, has also taken place. The amount of introgression from the Atlantic to the Pacific herring was variable between the various contact regions of the taxa. Most heavily introgressed fjord population in northern Norway has parallels in the boreal bivalves Macoma and Mytilus, for which similar repeated invasion history has been inferred.