Push-Me, Pull-You

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Science  18 Oct 2013:
Vol. 342, Issue 6156, pp. 290
DOI: 10.1126/science.342.6156.290-a

Natural selection is expected to shape phenotypes around an adaptive optimum. In cases where the trend is expected to be directional—bill lengthening in birds, for example—the trait under selection should move toward this optimum. In reality, however, traits often vacillate around it. One cause for this lack of movement is opposing selection. From a 25-year study of green-rumped parrotlets in Venezuela, Tarwater and Beissinger describe evidence for opposing selection as a causal factor. Females that reproduced early in the year produced more offspring; however, both the females and their offspring had lower survival in the following year, indicating a trade-off between selection for fecundity and that for viability. Further, these patterns were oppositely influenced by environmental conditions. Rainfall strongly selected for early breeding, whereas breeding density favored later breeding. Climate change could induce a declining positive-feedback loop wherein small numbers of offspring would be produced due to low rainfall, and this would lead to increased selection for viability and continued low recruitment.

Proc. Natl. Acad. Sci. U.S.A. 110, 10.1073/pnas.1303821110 (2013).

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