Experimental excursions on adaptive landscapes: density-dependent selection on egg size.

Area of Science:

• Ecology
• Evolutionary Biology
• Behavioral Ecology

Background:

• Density-dependent natural selection theories predict that intraspecific competition favors high-ability juveniles.
• Field studies are scarce due to challenges in manipulating natural population densities.
• Side-blotched lizards (Uta stansburiana) provide a model for long-term ecological research.

Purpose of the Study:

• To experimentally test density-dependent natural selection on egg size in a natural population.
• To investigate if egg size increases from early to late clutches due to competition.
• To assess how population density and age structure influence selection on egg size.

Main Methods:

• Conducted a decade-long field study on side-blotched lizards.
• Implemented environmental manipulations of hatchling density.
• Used phenotypic manipulations of egg size.
• Compared selection on egg size with and without early-hatchling competitors.

Main Results:

• Selection strength on egg size doubled for late-hatchlings in competitor-free environments.
• Density-dependent selection on egg size was experimentally demonstrated.
• Findings contradicted the hypothesis that egg size increases seasonally due to hatchling competition.
• Age/size asymmetries between hatchlings overrode egg size effects.

Conclusions:

• Competitive asymmetries, not just egg size, mediate selection pressures.
• Population density is a key factor in density-dependent natural selection.
• Simultaneous manipulation of traits and environmental agents offers robust experimental insights.
• Competition can drive oscillating selection pressures in ecological systems.