Mary had a littler lamb

摘要

Changes in a given species may be driven by evolution or by ecological changes. Discriminating between the two is fraught with difficulty.rnTim Coulson of Imperial College London and his colleagues say they have managed to tease out the different drivers for one such change. Soay sheep on the Scottish isle of Hirta, UK, have shrunk in size over the past 20 years, despite heavy sheep being more likely to survive when young.