Experimental Excursions on Adaptive Landscapes: Density-Dependent Selection on Egg Size

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 results indicate that competitive age or size asymmetries between early and late hatchlings can override within-cohort asymmetries due to egg size. We suggest that competition could be an important mediator of oscillating selection pressures in this and other systems. Finally, we discuss the utility of "double-level," simultaneous experimental manipulation of both phenotypic traits that are targets of selection (e.g., egg size) as well the environmental agents of selection (e.g., population density).

Uncoupling direct and indirect components of female choice in the wild

“…Females on high-quality territories laid their eggs an average of 5.7 days earlier than females on low-quality territories [ANOVA, F(1,13) ϭ 5.36, P ϭ 0.03], thereby increasing the likelihood that they would produce additional clutches of eggs (25) and their progeny would compete successfully and survive better than progeny that were laid later in the season (26). Females on high-quality territories also produced larger eggs for both sons and daughters than did females on low-quality territories [analysis of covariance for sons egg mass, F(1,11) ϭ 4.97, P ϭ 0.04, covariate for mother's mass P ϭ 0.09; for daughters egg mass, F(1,11) ϭ 7.25, P ϭ 0.02, covariate for mother's mass P ϭ 0.14; Fig.…”

Density-dependent competition and selection on immune function in genetic lizard morphs

“…Side-blotched lizards, U. stansburiana, are small (7-10 g) lizards that mature 1 year after hatching, and most adults survive only 1 season of reproduction. In our study area in the Coast Range of central California, adult population densities in certain years may approach 800 per hectare and juvenile densities 2,600 per hectare, which is probably among the highest for terrestrial vertebrate populations (11). As part of our long-term population studies (14), we captured and observed lizards during the reproductive seasons (late February to August) of 1998 and 1999.…”

“…Eggs obtained from the dams were incubated in a common laboratory environment with temperature and hydric conditions carefully controlled until hatching (7,11). Incubation in this common laboratory environment controlled for maternal effects in the field because of differences in oviposition sites between This paper was submitted directly (Track II) to the PNAS office.…”

“…dams (7). In addition, the sizes of some eggs were manipulated upwards (follicle ablations) or downwards (yolk removals), whereas others served as unmanipulated controls, as part of our long-term, ongoing experimental field study (7,11,14). These experimental manipulations controlled for maternal effects attributable to differences in the amount of yolk deposited in the eggs (7).…”

“…This is an annual species throughout most of its range, and it is characterized by very high densities of both adults and juveniles. These high densities lead to territorial interactions between neighbors and have negative effects on progeny and adult survival (10,11). In several lizard species, including side-blotched lizards, males and females occur in discrete and heritable color morphs that differ markedly with respect to behavior, physiology, and lifehistory traits (12)(13)(14)(15)(16).…”