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Very brown sheep have a dark side

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11:00am, January 30, 2008
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Darker sheep are bigger, but now we learn why they're not better.

Soay sheep living wild on the Scottish island of Hirta come in two colors: light brown and dark brown. Researchers have known that the dark sheep are bigger than the light ones. Extra heft translates into an advantage in winter survival and reproduction. "We would have expected [the dark sheep] to have increased in frequency over a 20-year time series," says Jon Slate of the University of Sheffield in England.

Not so, Slate and his collaborators say in the Jan. 18 Science. Biologists have kept records on the Hirta population since 1985, and big dark sheep have not been taking over the population. Analyzing the ups and downs of colors shows that dark-coated sheep are becoming less common.

The team analyzed color-inheritance patterns in detail, using methods similar to those of geneticists searching for disease genes. It turns out that large size isn't the only trait passed along with the dark coat. Some other genetic material piggybacks and undermines the size advantage by decreasing success in reproduction in sheep with two copies of the dark-coat gene variant, the researchers report. Slate says the work highlights the importance of looking for tightly associated genes when trying to predict the evolutionary fate of a trait.

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