Field experiments that measure natural selection in response to manipulations of the selective regime are extremely rare1, even in systems where the ecological basis of adaptation has been studied extensively. The adaptive radiation of Caribbean Anolis lizards has been studied for decades2,3,4,5, leading to precise predictions about the influence of alternative agents of selection in the wild. Here we present experimental evidence for the relative importance of two putative agents of selection in shaping the adaptive landscape for a classic island radiation. We manipulated whole-island populations of the brown anole lizard, Anolis sagrei, to measure the relative importance of predation versus competition as agents of natural selection. We excluded or included bird and snake predators across six islands that ranged from low to high population densities of lizards, then measured subsequent differences in behaviour and natural selection in each population. Predators altered the lizards’ perching behaviour and increased mortality, but predation treatments did not alter selection on phenotypic traits. By contrast, experimentally increasing population density dramatically increased the strength of viability selection favouring large body size, long relative limb length and high running stamina. Our results from A. sagrei are consistent with the hypothesis6 that intraspecific competition is more important than predation in shaping the selective landscape for traits central to the adaptive radiation of Anolis ecomorphs.
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We thank T. B. Smith for suggestions about experimental design, and B. Calsbeek and A. Gasc for help with predator manipulations. M. C. Duryea, S. Kuchta, M. Logan, M. Najarro and D. Urbach helped to clarify the manuscript. This research was conducted under permits from The Bahamas Ministry of Agriculture and approval from the Dartmouth College Institutional Animal Care and Use Committee. An award from the National Science Foundation to R. Calsbeek, and funding from Dartmouth College, provided financial support.
The authors declare no competing financial interests.
This file contains Supplementary Methods, Supplementary Tables S1-S2 and Supplementary Figures S1-S2 with legends. Supplementary Table 1 was amended on 3 June 2010. (PDF 550 kb)
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Calsbeek, R., Cox, R. Experimentally assessing the relative importance of predation and competition as agents of selection. Nature 465, 613–616 (2010). https://doi.org/10.1038/nature09020
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