Infection Protection and Natural Selection

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Science  29 Oct 2010:
Vol. 330, Issue 6004, pp. 602-603
DOI: 10.1126/science.1198303

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Why are infectious diseases still the major causes of animal mortality, despite the strong and ever-present evolutionary pressure to eliminate traits that make individuals susceptible to infection? Why hasn't natural selection weeded out weak immune systems? An emerging and rapidly growing field, known as ecological immunology or ecoimmunology, is trying to answer these questions by investigating how variation in immune response in free-living animal populations affects fitness. On page 662 of this issue, Graham et al. (1) provide an illuminating answer for feral sheep living on the Scottish island of Hirta in the St. Kilda archipelago. The sheep have varying blood levels of antibodies, and individuals with higher levels appear to confront a fitness trade-off: Although a strong immune response helps adult females survive harsh winters, it also reduces reproduction.