Evol Ecol Res 7: 287-302 (2005)     Full PDF if your library subscribes.

Soft and hard selection on plant defence traits in Arabidopsis thaliana

Joanna L. Kelley,1 John R. Stinchcombe,2* Cynthia Weinig3 and Johanna Schmitt 2

1Department of Genome Sciences, University of Washington, Seattle, WA 98195,  2Ecology and Evolutionary Biology, Brown University, Box G-W, Providence, RI 02912 and  3Department of Plant Biology, University of Minnesota, 1445 Gortner Avenue, St. Paul, MN 55108, USA

Author to whom all correspondence should be addressed.
e-mail: john_stinchcombe@brown.edu


Whether population regulation is global or local determines if hard or soft selection, respectively, is operating. Differences between hard and soft selection, in turn, affect the maintenance of genetic variation and the mean strength of natural selection in heterogeneous environments. Here we investigated whether there were differences in the strength of soft and hard selection on plant defence traits in a spatially replicated field experiment with Arabidopsis thaliana. We estimated selection gradients for basal branch number and susceptibility to apical meristem herbivory in 10 spatial environments of an Arabidopsis field experiment. Net patterns of soft selection across these 10 spatial environments were calculated as weighted averages of the selection gradients, weighted by the frequency of each environment. Hard selection was estimated in a similar fashion, except that selection gradients for each spatial environment were first multiplied by a fitness ratio that assigned more or less weight to selection gradients depending on whether local mean fitness in that environment was greater or less than global mean fitness across all environments. In general, soft selection was stronger than hard selection, possibly because selection on plant defence traits is relaxed in environments that otherwise lead to high fitness. To evaluate the sensitivity of our results to the frequency of spatial environments, we repeated our analyses under several different hypothetical scenarios in which we varied the cumulative frequency of the environments that led to highest fitness. In these analyses, we found that the mean strength of hard and soft selection, and the relative magnitude of the two estimates, varied greatly depending on the frequency of high fitness environments. These analyses also revealed that estimates of the overall pattern of selection in spatially heterogeneous environments can be very sensitive to the frequency of individual, unusual environments – for example, environments with above average fitness but local patterns of selection dramatically different from other above average fitness environments. We suggest that more empirical effort be devoted to characterizing the frequency of selective environments in natural systems, although that is likely to be extremely challenging.

Keywords: apical meristem damage, frequency of selective environments, genotypic selection analysis, hard selection, herbivory, soft selection, spatial variation, tolerance.

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        © 2005 Joanna L. Kelley and John R. Stinchcombe. All EER articles are copyrighted by their authors. All authors endorse, permit and license Evolutionary Ecology Ltd. to grant its subscribing institutions/libraries the copying privileges specified below without additional consideration or payment to them or to Evolutionary Ecology, Ltd. These endorsements, in writing, are on file in the office of Evolutionary Ecology, Ltd. Consult authors for permission to use any portion of their work in derivative works, compilations or to distribute their work in any commercial manner.

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