Counting Sheep

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Science  09 Jun 2006:
Vol. 312, Issue 5779, pp. 1442
DOI: 10.1126/science.312.5779.1442b

The environment can be a powerful force in evolution, as the great mass extinctions across geological time testify. Yet classical models of the genetics of populations often assume the simplifying condition of a constant environment, begging the question of what happens to the heritability and selection of specific traits in times of change. Details of the phenotype of Soay sheep—first introduced to the Scottish archipelago of St. Kilda in the Bronze Age and to the main island of Hirta in 1932—have been collected since 1985 and provide a case study of microevolution through changeable times.

Wilson et al. analyzed the birth weight of Soay sheep across a 20-year period during which the sheep experienced both low and high mortality rates associated with changing environmental circumstances. Birth weight is a heritable trait that is under potentially strong selective pressure, with larger lambs having a better chance of survival. Under harsh conditions, the researchers find that their models are consistent with a strong selection for increased birth weight among lambs, which is also associated with a low genetic variation. Favorable conditions result in a reduced selection on birth weight. Thus, for this trait in Soay sheep, the environment acts as a constraint on the microevolutionary potential of the population. — GR

PLoS Biol. 4, e216 (2006).

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