Eggs on the Rise

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Science  17 Feb 2006:
Vol. 311, Issue 5763, pp. 919
DOI: 10.1126/science.311.5763.919c

A bird's clutch size—the quantity of eggs laid during a nesting period—is a central feature of a bird's life history, but has presented an evolutionary conundrum. Although studies of bird species have predicted the existence of positive selection for increasing clutch size over time, such increases have failed to materialize during long-term observation, perhaps because of constraints imposed by correlated environmental factors that also affect fitness.

In a 25-year study of mute swans, Charmantier et al. observed not only the expected directional selection for increasing clutch size, but also an actual increase, of 0.35 standard deviations, across the population. Reduced predation and increased food supply over the course of the study may have fostered the increase. Because the authors kept track of the pedigrees of all of the individuals in the study, they garnered strong evidence that these changes were genetic rather than phenotypic, and hence that a clear microevolutionary change took place over the course of a quarter century. — AMS

Am. Nat. 167, 10.1086/499378 (2006).

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