Azara Blog: Some Scottish sheep are apparently getting smaller

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Date published: 2009/07/02

The BBC says:

Climate change is causing a breed of wild sheep in Scotland to shrink, according to research.

Scientists say milder winters help smaller sheep to survive, resulting in this "paradoxical decrease in size".

Classic evolutionary theory would predict that wild sheep gradually get bigger, as the stronger, larger animals survive into adulthood and reproduce.

Reporting in Science journal, the team says this shows the "subtle interplay" between evolution and the environment.
"In the past, only the big, healthy sheep and large lambs that had piled on weight in their first summer could survive the harsh winters on Hirta," said Professor Coulson.

Because of climate change, he explained, grass for food is now available for more months of the year on the island.

"Survival conditions are not so challenging - even the slower growing sheep have a chance of making it, and this means smaller individuals are becoming increasingly prevalent in the population," he said.

The team also found that younger sheep tended to give birth to smaller lambs - a phenomenon they termed "the young mum effect".

This effect, said Professor Coulson, combined with environmental changes had "overriden what we would expect through natural selection".

To say that there is "subtle interplay between evolution and the environment" is rather misleading. It is not "subtle" at all. And environmental changes have not "overriden what we would expect through natural selection". The environment is one of the main drivers of evolution. Genes that might be useful in one environmental context might not be useful (or so useful) in another.

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