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Summary: 1396
2000 The Society for the Study of Evolution. All rights reserved.
Evolution, 54(4), 2000, pp. 13961403
EXPERIMENTAL EXCURSIONS ON ADAPTIVE LANDSCAPES: DENSITY-DEPENDENT
SELECTION ON EGG SIZE
ERIK SVENSSON1,2 AND BARRY SINERVO1
1Department of Biology, Organismal and Population Biology, Earth and Marine Sciences Building, University of California,
Santa Cruz, California 95064
Abstract. Theories of density-dependent natural selection suggest that intraspecific competition will favor juveniles
of high competitive ability. Empirical evidence has been provided from laboratory selection experiments, but field
studies are lacking due to the logistical difficulties of experimentally manipulating population densities in natural
settings. Here, we present data from a decade-long experimental field study of side-blotched lizards, Uta stansburiana
that overcomes these difficulties. We tested the hypothesis that density-dependent natural selection causes egg size
to increase from early to late clutches in this and many other species. Using a novel combination of environmental
manipulations of hatchling density and phenotypic manipulations of egg size, we demonstrate that the nature of
selection on egg size changes dramatically in the absence of older competitors. The strength of selection on egg size
among later-clutch hatchlings released in areas without competitors from early clutches became almost doubled in
magnitude, compared to that among hatchlings released in the presence of older competitors. These experimental
findings demonstrate density-dependent natural selection on egg size; however, they contradict the classical idea that
egg size increases during the reproductive season because of competition between early and late hatchlings. The
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