Accelareated morphological evolution in insular mammals: is it true or not? The case of the Vancouver Island marmot [L'évolution morphologique accélérée des mammifères insulaires : est-elle vraie ou non? Le cas de la marmotte de l'île de Vancouver].

Résumé

On islands mammalian morphological evolution is often accelerated and large species tend to become smaller. These trends are attributed generally to rapid adaptation to a new environment after isolation and possibly to genetic drift. The Vancouver Island marmot, the only marmot species endemic to an island, is a rare endangered species, which likely became isolated at the end of the last glaciation. Despite its young evolutionary age and genetic similarity with continental populations of the hoary marmot, it has distinct alarm calls and fur colour. Recent studies have shown that its skull morphology is also highly distinctive. However, how well this fits into the bigger picture of evolution of insular mammals has not yet been examined. Also, the possibility that distinctiveness in modern specimens is an artefact of sampling error has not yet been excluded. Using skull measurements of modern sand prehistoric Vancouver Island marmot we investigated: a) what the evidence is for genetic drift/natural selection in morphological evolution; b) whether size has changed as predicted by the island rule; c) whether unique aspects of shape are confirmed in modern specimens and occur in subfossils dating from 1000-2500 years ago. We found: a) weak evidence for an exclusive role of genetic drift; b) no appreciable change in size over ~2500 years; c) unique shape traits which likely evolved shortly after isolation. This study is relevant to our understanding not only of evolutionary processes but also of species responses to isolation, habitat fragmentation and destruction due to human activities and climate change.