A colony of penguins that has bred at the same site in Antarctica for thousands of years has provided scientists with a rare insight into how a change in the climate can speed up the course of evolution.
Researchers analysed ancient fragments of DNA from the remains of penguins that have been buried at the site for up to 6,000 years and compared them to the DNA of living members of the same colony.
The comparison has offered a snapshot of small-scale evolutionary changes to the genetic sequence of the penguins' DNA that have occurred without any obvious changes to the appearance or behaviour of the birds.
Scientists led by David Lambert of Massey University in Auckland, New Zealand, found that there had been significant changes to the DNA of the penguin colony that have probably arisen when huge icebergs break off from the Antarctic ice shelf.
In 2001, icebergs nearly isolated Antarctica's penguins from their feeding grounds in the open sea. Many penguins had to walk for miles on ice instead of swimming to feed.
The melting icebergs - which can be the size of English counties - can interfere with the normal movements and breeding patterns of the penguins, causing a shift in the frequency of their genes, the scientists say in the journal Proceedings of the National Academy of Sciences.
They believe that many of these "mega-icebergs" have broken away over the past few thousand years and each resulted in small-scale evolutionary changes.
"Microevolution is regarded as changes in the frequencies of genes in populations over time," the scientists write. "Ancient DNA technology now provides an opportunity to demonstrate evolution over a geological time frame and to possibly identify the causal factors in any such evolutionary event."
The scientists studied the adelie species which breeds in high-density colonies ranging in size from about 200 individuals to 170,000 breeding pairs.
A feature of this particular penguin is that individuals return to the same nesting sites year after year unless other factors interfere, such as the sudden appearance of a large iceberg.
Female adelie penguins usually lay two eggs each breeding season but about one in four chicks dies before they are able to fledge, which has resulted in a large build up of remains that become semi-fossilised in the dry, cold climate. "Droppings, feathers, egg fragments and other penguin remains, mixed with sand, gravel and pebbles, constitute distinct soil horizons," the scientists say.
Because adelie penguins show such a strong tendency to return to the same breeding site used by their parents, the scientists believe that the DNA they extract from the fossilised remains derives from the direct ancestors of penguins living in the colony today.
The region of the DNA studied by the scientists is not actually responsible for any genes so any changes are due to a process of genetic "drift" rather than natural selection.