Why are the dark sheep on the decline in a wild herd in Scotland, even though they're bigger than the light-colored sheep? It all comes down to genes, researchers reported Thursday.
The population of the wild Soay sheep on the isolated island of St. Kilda has been virtually unchanged over the past 4,000 years, giving modern-day researchers a unique view into natural selection and evolution.
About three-quarters of the sheep in the herd are dark. but their numbers have dwindled over the past 20 years — which is a puzzle for scientists. The dark sheep tend to be bigger, which would theoretically give them an evolutionary advantage to survive the island's harsh winters.
"If being big is good, and dark sheep are bigger, we would expect the frequency of dark sheep to increase," said Jon Slate, a researcher at the University of Sheffield, who worked on the study published in Friday's issue of the journal Science. "This presents an evolutionary problem."
To address the riddle, the scientists ran a statistical analysis of the genes that determined color. Like all animals, sheep inherit one version of each gene from each parent — and these sheep can inherit either a gene for a dark brown coat or a gene for a white coat from each parent.
The researchers determined that the gene for a dark coat was dominant — thus, dark sheep could carry either two dark genes, or a dark gene and a light gene. They also found that the dark gene was usually accompanied by a set of genes that increase size but decrease reproductive success. In contrast, the light gene was usually packaged with other genes for smaller size but better lifetime fitness.
As a result, sheep with one copy of the dark gene and one copy of the light gene were large and dark (from the dark gene), but also had high reproductive success (from the light gene). Sheep with two copies of the dark gene were larger still, but were less fit for reproduction. Sheep with two copies of the light gene were small but more fit.
The two types of dark sheep — dark-dark and dark-light — were visually indistinguishable. But the dark-light sheep were more likely to reproduce, and in the process had an opportunity to pass along their light-coat gene.
"We have an example of a counterintuitive trend and show that it is actually still consistent with evolutionary theory," he told Reuters in a telephone interview. "It helps explain why predicted evolutionary trends are sometimes not observed."
The researchers determined reproductive fitness based on the herd's population statistics, but they did not identify precisely why the genes inherited along with the light-coat gene made them more successful.
This report includes information from Reuters and msnbc.com.