The role of optimizing selection in natural populations.
In this review, it is argued that (1) optimizing selection is a significant process affecting the distribution of some types of phenotypic traits and (2) the variety of processes that give rise to an optimizing selection pressure, and the problems inherent in its study have limited the understanding of what is seen when selection apparently acts on a trait in an optimizing fashion. Optimizing selection is defined as viability of fertility selection against extreme phenotypes that are expressed in spite of any developmental buffering, and refers only to metric traits or meristic traits with many values. Its relationship with stabilizing selection, the importance of recognizing correlated traits, horizontal vs. vertical studies, deceptive fitness functions and statistical issues are examined. Five aspects of optimizing selection are surveyed: multiple ecological factors acting directly on a trait but in opposite directions, direct and indirect selection acting antagonistically, pleiotropic overdominance, and selection against extreme phenotypes and removal of phenotypic load.