Natural selection in island sheep

No one knows for sure when or how they got there, but Soay sheep (Ovis aries) arrived on the isolated, wind-swept Scottish island of Soay sometime during the Bronze Age, thousands of years ago. In the 1930s, a flock was relocated to Hirta, an adjacent island in the St. Kilda archipelago, after villagers evacuated to the mainland. Since then, the sheep have been left mostly to their own devices, save for ongoing encounters with scientists keen to test a range of evolutionary hypotheses on this unique population.

Alastair Wilson, Loeske Kruuk, and colleagues took advantage of a 20-year study of birth weight in Soay sheep--which covered nine generations and significant climatic fluctuations--to examine the impact of environmental quality on the strength of selection and the amount of genetic variation associated with a fitness-related trait. They now report--in the open-access journal PLoS Biology--that the extent to which birth weight is heritable--that is, the variance caused by maternal genetic effects--increased along with environmental quality.

Evolution should occur when selection acts on a trait, such as birth weight, that has a genetic basis. Birth weight is closely linked to juvenile mortality, with larger lambs having higher survival and higher fitness. The first six months of life are critical; lamb deaths during this crucial window account for nearly a quarter of all deaths. Even though birth weight has all the raw materials for evolution--a positive correlation between birth weight and survival, and a heritable basis for variation, attributed to maternal genetic effects--previous studies found no evidence for evolution of the trait. Since annual birth weight had not increased over the 20 years, the researchers hypothesized that a variable environment could limit its micro-evolution by limiting heritability, selection, or both. Wilson et al. found evidence that that this is indeed the case.

To study the micro-evolutionary d

Contact: Natalie Bouaravong
Public Library of Science

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