Mechanistic Analysis of Natural Selection and a Refinement of Lack's and Williams's Principles.

Results from several physiologically based manipulations were synthesized to investigate two selective trade-offs involving offspring number versus offspring quality and costs of reproduction in an annual lizard Uta stansburiana. Lifetime reproductive success of experimentally size-altered progeny was studied to address the offspring number and offspring quality trade-off. Causes of natural selection on adult reproductive costs were assessed with three complementary manipulations of clutch size, egg size, and total clutch mass. Selective trade-offs between offspring size and number arose from two opposing episodes of directional selection. Fecundity selection favored female parents that laid large clutches of small offspring, but fecundity selection was balanced by survival selection that favored large offspring. Thus, the offspring number and quality trade-off had a strong stabilizing effect on mean egg size across generations. However, strength and direction of selection arising from adult reproductive costs varied among years. Because reproductive traits were heritable ([Formula: see text]), selection on adult reproduction led to a large evolutionary response to natural selection. Patterns of selection detected in natural phenotypic variation were largely corroborated by phenotypic manipulations. However, maturational costs of reproduction that were detected with phenotypic manipulations were missed by traditional selection analysis of natural phenotypic variation.