Featured Science Paper
Evolutionary dynamics at high latitudes: speciation and extinction in polar marine faunas
Although the polar regions are largely devoid of life on land, the same is not true in the sea. The polar seas contain a wide variety of plants and animals and the question is often asked, “how long have they been there?” There is evidence from both the fossil record and the exciting new field of molecular genetics (where evolutionary trees are built from the DNA of living organisms) that many polar organisms are indeed ancient. We believe that two great evolutionary events have largely shaped the composition of modern polar marine faunas. The first of these is the mass extinction event 65 million years ago at the Cretaceous-Tertiary boundary, and the second is the onset of widespread glaciations some 25-30 million years ago.
So how did marine organisms living on the polar continental shelves survive repeated glaciations over the last few million years? We believe that, quite possibly, there were refugia in both polar regions where organisms could have survived even the most intense periods of glaciation. These may have simply been open water polynya or areas of continental shelf that do not bear the tell-tale marks of glacial erosion. Or it may be that many shallow water organisms escaped the ice by migrating into the deep sea, only to move back into shallower waters when the ice retreated.
We still have much to learn about the interaction between glacial climates and biological evolution. Recent sediment cores from the Ross Sea have revealed at least 38 complete glacial cycles within the last five million years alone and these must have continually altered the nature of available habitats for all marine organisms. Although we tend to think of such cycles as agents of extinction, it is quite possible that they may also have promoted the evolution of new species by continually isolating and then reuniting discrete populations. Far from obliterating all life in the shallow seas, the polar ice sheets may in fact have been significant engines of biodiversity.
Clarke, A. and Crame, J.A.
Philosophical Transactions of the Royal Society of London B, 365, 3655-3666