Bye, bye dark sheep due to genes

Light (L) and dark-coated Soay rams battle for dominance in St Kilda, Scotland, in this undated handout photograph made available in London January 17, 2008. The declining number of dark sheep among a wild herd in Scotland comes down to genes, researchers said on Thursday.

LONDON (Reuters) - The declining number of dark sheep among a wild herd in Scotland comes down to genes, researchers said on 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 dwindling numbers have puzzled scientists. This is because dark animals tend to be bigger, which should give them an evolutionary advantage to survive 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 the journal Science.

"This presents an evolutionary problem."

To solve the riddle the scientists analyzed versions of 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 coat or a gene for a white coat from each parent.

The researchers determined that the gene for a dark coat is dominant -- dark sheep carry either two dark genes or a dark gene and a light gene.

But they also found that having a light gene boosts fitness so the best combination in evolutionary terms for the sheep is a mix of genes that produce a dark coat.

This explains the decline of dark sheep because those with a pair of dark genes are the least fit, even though they are big, Slate said. The researchers do not know why the light gene confers fitness, he added.

"We have an example of a counterintuitive trend and show that it is actually still consistent with evolutionary theory," he said in a telephone interview. "It helps explain why predicted evolutionary trends are sometimes not observed."

(Reporting by Michael Kahn; Editing by Maggie Fox)