Massive chunks of ice that blocked migration routes sped up the evolution of some Antarctic penguins. That's one theory biologists are putting forward to explain a striking number of genetic differences between modern members of this group and their 6000-year-old ancestors, whose DNA has been recovered from an ancient Antarctic nesting site.
Ancient DNA is a potential gold mine for biology. It allows scientists to directly trace evolution over thousands of years. A good source of ancient DNA is the Antarctic nesting sites of Adélie penguins. The birds return to the same place each summer to mate, and because about one in four chicks die on the spot, a rich layered record of the populations goes back for millennia. Thanks to the extremely cold and dry environment, their DNA is preserved like samples in a giant laboratory freezer.
L David Lambert, a molecular biologist at Massey University in Palmerston North, New Zealand, has been studying the DNA of Adélie penguins for nearly a decade to understand the birds' recent evolution. In 2002, Lambert's team successfully extracted and sequenced ancient penguin mitochondrial DNA (ScienceNOW, 22 March 2002) and found a rate of evolution far higher than that reported for other species. Now the team has extracted the penguins' nuclear DNA, which provides a more sensitive measure of evolutionary change because it is passed down from both mother and father (mitochondrial DNA passes only from the mother).
The penguins have racked up a surprising amount of genetic change over the past 6 millennia, the team reports online this week in Proceedings of the National Academy of Sciences. When the researchers compared nuclear DNA from the bones of 15 6000-year-old penguins to 48 living penguins at the same nesting site, four of nine short stretches of repetitive DNA called microsatellites had grown longer, and two had shrunk. Such dramatic changes were not expected for such stable, isolated populations, and this is the first time they have been observed within a single species, say the authors. Lambert believes events like a mega-iceberg breaking off from the Ross Ice Shelf in 2001, which blocked the penguins' normal migration route, may have happened many times over the millennia. This would have periodically forced separate populations to mate together at new sites, injecting fresh genetic variation into the gene pool.
The experimental system is "beautiful," says Axel Meyer, a biologist at the University of Konstanz, Germany. But the same process that spurred penguin diversity could drive them to extinction. Global warming will likely break off many more mega-icebergs and spur other dramatic changes to the Antarctic environment, Meyer says, which could seriously limit the penguins' nesting options.