Experimental excursions on adaptive landscapes: Density-dependent selection on egg size

Theories of density-dependent natural selection suggest that intraspecific competition will favor juveniles of high competitive ability. Empirical evi dence has been provided from laboratory selection experiments, but field st udies are lacking due to the logistical difficulties of experimentally mani pulating population densities in natural settings. Here, we present data fr om a decade long experimental held study of side-blotched lizards, Uta stan sburiana that overcomes these difficulties. We tested the hypothesis that d ensity-dependent natural selection causes egg size to increase from early t o late clutches in this and many other species. Using a novel combination o f environmental manipulations of hatchling density and phenotypic manipulat ions of egg size, we demonstrate that the nature of selection on egg size c hanges dramatically in the absence of older competitors. The strength of se lection on egg size among later-clutch hatchlings released in areas without competitors from early clutches became almost doubled in magnitude, compar ed 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 inc reases during the reproductive season because of competition between early and late hadchlings. The results indicate that competitive age or size asym metries between early and late hatchlings can override within-cohort asymme tries due to egg size. We suggest that competition could be an important me diator of oscillating selection pressures in this and other systems. Finall y, we discuss the utility of "doubie-level," simultaneous experimental mani pulation of both phenotypic traits that are targets of selection (e.g., egg size) as well the environmental agents of selection (e.g., population dens ity).