Sources of Variation in Plant Reproductive Success and Implications for Concepts of Sexual Selection

Abstract

There is growing evidence of nonrandom reproductive success in plants. The potential evolutionary effect of these patterns depends on the extent to which they reflect nonrandom transmission of genes between generations. A recent tendency has been to examine these patterns in the context of sexual selection. We present an experimental protocol designed to yield more information on nonrandom transmission in general and on sexual selection as a special case. The mainstays of the protocol are twofold. First, reproduction is separated into pollination, fertilization, and seed-maturation phases, in which functional processes associated with the genders are known and in which the genders can be manipulated. Second, sources of variation in reproductive success are identified by systematically repeating a reciprocal-crossing design with treatments that alter the intensity with which males and/or females can interact. We discuss appropriate experimental and analytic techniques, especially designs for analyses of variance. Our emphasis is primarily on identifying genetic sources of variance. A selective literature review illustrates how some of our design elements have been applied in the three stages of reproduction. We discuss some of the statistical and experimental difficulties that plants pose for the study of nonrandom transmission, as well as some of the advantages of working with plants. Even when the difficulties are overcome, there is considerable disagreement about the biological interpretation of results, particularly with regard to sexual selection. Disagreements result from differential reliance on arguments about past selective events, difficulty in identifying heritable male traits on which selection can act, and limits on inference imposed by experimental constraints. Plant biologists will continue to disagree on these matters. We hope that they will not be overly concerned with fitting plants into animal-derived models of sexual selection and will concentrate instead on systematically documenting how evolutionary mechanisms work on plant reproduction via nonrandom genetic transmission.