Researchers have solved a mysterious problem that seemed to defy Darwinian logic.

Sheep living on an island off Scotland with dark coats are bigger than their fellow sheep that have light coats. This should make them more likely to survive the bitter Scottish winters and thus it might seem likely that dark sheep would come to make up a greater proportion of the woolly population.

However the proportion of dark sheep is actually declining. “If being big is good and dark sheep are bigger we would expect the frequency of dark sheep to increase. This presents an evolutionary problem,” Jon Slate, a researcher at the University of Sheffield, told Reuters.

In the latest issue of Science Slate and colleagues have solved this problem. Bigger, they found, isn’t always better. They looked at a small portion of the Soay sheep genome which contains the coat colour gene and some others that are usually inherited with it. They found that some of the other genes that occur with the dark coat gene have negative effects on health – such as reduced reproductive success – that outweigh the benefits of being larger (research abstract, press release).

“This was an enigma, but we’re excited to have solved it. The lighter sheep were benefiting from genes that made them both more fit and more able to have offspring. That’s why we’re seeing the population of the darker sheep go down. It’s evolutionary theory in action,” says Slate (in the Guardian).

So Darwin can rest easy. In the words of the Scotsman: “A flock of sheep living on a remote Scottish island do not contradict Darwin’s theory of evolution, a scientific study has found.”

But does this mean that eventually all the sheep will be blonde? Maybe not. It seems some sheep have one copy of the dark coat gene and one copy of the light coat gene. These are not as large as the dark sheep but not as small as the light sheep. They also have more reproductive success than the dark sheep, although not as much as the light sheep.

Maybe then there is hope for the survival of the dark coat gene. This is good; every family needs its black sheep after all.

Image: Soay rams in the annual rut on St. Kilda / courtesy of Ian Stevenson