The researchers had all the right elements in place to watch evolution in action:
a species that inhabits the same, very limited geographical area generation after generation;
a significant number of deaths of individuals in a specific location;
a cold and dry climate that preserves bones and DNA; and
easy-to-identify and -date soil layers.
The scientists examined nine genetic areas inherited from both parents. The zones occur in noncoding regions of the penguins' total genetic makeup, or genome, that do not make proteins, Lambert explained.
"The usual expectation is that if a region is noncoding, it will not have an effect on the organism in terms of its [form and structure], behavior, and so on," he said. "Hence it is not likely that [the creature] will be acted upon by natural selection, because individuals with different DNA sequences are not likely to have different abilities to survive and reproduce."
The researchers found significant changes in the birds' allele frequencies over time and demonstrated for the first time a difference in the genetic composition of penguin populations separated by geologic time.
Natural selection, the scientists concluded, probably doesn't explain the genetic changes. Rather, the researchers have identified large icebergs as the probable cause.
In 2001 a large iceberg broke from Antarctica's Ross Ice Shelf, drifting to an area with large concentrations of Adélie penguin colonies. This blocked the penguins' normal migration routes, which they swim to return to their birthplace breeding grounds.
As a result, some of the colonies produced no offspring that year. The phenomenon has occurred often over time. Estimates are that as many as 200 such icebergs have calved during the past 10,000 years.
Researchers say it was almost certainly these geological changes and their effect on migration, and not natural selection, that induced the penguins' gene changes and caused microevolution.
"We are convinced that the changes are due, largely, to the effects of mega-icebergs, which cause breeding birds to move form their natal colonies to other breeding colonies," Lambert said. "This has caused the gene frequency changes over time, i.e., evolution."
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES