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Predicting Microevolutionary Responses to Directional Selection on Heritable Variation

Peter R. Grant and B. Rosemary Grant
Evolution
Vol. 49, No. 2 (Apr., 1995), pp. 241-251
DOI: 10.2307/2410334
Stable URL: http://www.jstor.org/stable/2410334
Page Count: 11
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Predicting Microevolutionary Responses to Directional Selection on Heritable Variation
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Abstract

Microevolution of quantitative traits in the wild can be predicted from a knowledge of selection and genetic parameters. Testing the predictions requires measurement of the offspring of the selected group, a requirement that is difficult to meet. We present the results of a study of Darwin's finches on the Galapagos island of Daphne Major where this requirement is met. The study demonstrates microevolutionary consequences of natural selection. The population of medium ground finches, Geospiza fortis, experienced size-selective mortality during a drought in 1976-1977; large birds with deep beaks survived better than small birds. During another drought, 1984-1986, the population experienced selection in the opposite direction on beak traits. Changes in food supply were the apparent causes of selection on beak traits in both episodes. As expected from the high heritabilities of all measured traits, the effects of selection were transmitted to the next generation. Evolutionary responses to both episodes of selection were quantitatively well predicted in general. This allows us to conclude that, to a first approximation, targets of selection were identified correctly, and genetic parameters were correctly estimated. Nevertheless, not all responses of individual traits were equally well predicted. A search for possible reasons for the largest discrepancies revealed evidence of bias caused by environmental effects on growth and adult size of some traits, as well as possible selection on the offspring generation before their measurement. These findings illustrate an important assumption in the study of microevolution: that the environments experienced during growth to maturity by the parental and offspring generations are the same, for otherwise a measured difference between generations may have a partly environmental cause, thereby giving a misleading estimate of the evolutionary response to selection. Simple extrapolations from observed selection to long-term evolution may underestimate the total force of selection involved if it oscillates in direction or acts on the environmental variance.

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