How Climate Change May Shrink Species

The people of Soay Island, off the west coast of Scotland, have notice something strange. Over the years, their sheep have begun to shrink, as I wrote in 2009:

Why? In short, because of climate change. Generally, the sheep's life cycle goes like this: they fatten up on grass during the fertile, sunny summer; then the harsh winter comes, the grass disappears and the smallest, scrawniest sheep die off, while their bigger cousins survive. That's how you end up with big sheep, which — according to Darwin's laws of natural selection — will pass on their big genes to the next generation.(See pictures of sheep and other animals.)

But over the past 25 years, the average Soay Island wild sheep has decreased in size, according to a report in the July 2 issue of Science by a team of researchers led by Tim Coulson of Imperial College London. Thanks largely to global warming, the winters on Soay Island are becoming shorter and milder. That makes food more abundant and allows some of the smaller, more vulnerable and younger sheep to survive. Then they go on to have offspring that tend to be small themselves — and have a better chance of survival because of the increasingly mild winters. "The environmental and evolutionary processes are intertwined," says Coulson "There's still natural selection, but it's not leaving as big a signature as it used to. There's still a disadvantage to being small, but not as much."  More Read