Ancient Penguin DNA Reveals Microevolution on Ice
for National Geographic News
|November 8, 2005|
Using perfectly preserved, ancient DNA, scientists have demonstrated
microevolution in a single species over a span of some 6,000 years.
The researchers examined well-preserved bones of Adélie penguins (Pygoscelis adeliae) found in Antarctica and compared them to the birds' living descendants.
The scientists discovered small changes in gene frequency, the relative percentage of an allele compared to nearby genes. Alleles are different forms of a gene (the code for blue or brown eyes, for example) that are inherited from each parent and that occupy a given space on a chromosome.
The evolutionary changes in the penguin genes have several possible causes. The most likely may be a recurring natural event in the birds' environmentthe mega-icebergs that intermittently alter the ways the penguins migrate and breed.
Biology professor David Lambert said that while many people might assume that natural selection propels evolution, "virtually all biologists define evolution as changes in gene frequencies over time."
Lambert, a researcher at the Institute of Molecular Biosciences at Massey University in Auckland, New Zealand, co-authored the new study. The findings were published yesterday in the Proceedings of the National Academy of Sciences Online Early Edition.
Until now, such gene frequency changes have only been measured over very short time scales. Some studies, for example, show contemporary changes in isolated populations of salmon or bird species.
But the preserved DNA of ancient Adélie penguins provided researchers with the opportunity for a much longer-term look at the process. The fossil bones contain the highest quality ancient DNA ever discovered, study authors say.
Adélie penguins breed in large colonies, ranging from 100 to 170,000 breeding pairs. The animals spend most of the winter on pack ice and invariably return to their birthplace to breed in ice-free coastal areas in summer.
Females usually produces two eggs, but almost a quarter of the chicks perish. This leaves large quantities of bones below the birds' nests that are preserved in distinct layers. Radiocarbon analysis can date the layers.
Put these circumstances together, and scientists are left with a bounty of DNA of known age andmost importantlyindividual ancestry.
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."
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