The Functional Significance and Fitness Consequences of Heterophylly.

Asymmetry along the longitudinal axis of a plant can be created by heterophylly, the production of leaves of different size or shape by an individual. Heterophylly is a widespread phenomenon ranging in degree from subtle variation in leaf size to striking differences in the size and shape of leaves produced by a single plant. Because leaf size and shape influence critical leaf functions, heterophylly may have significant consequences for plant fitness. To date, very few studies have directly addressed whether differences in leaf phenotypes produced by a single plant do result in differences in their functional properties or how any such differences affect individual survival and reproduction. I suggest some general approaches to determining the ecological and evolutionary significance of heterophylly and illustrate these approaches with two case studies from my work. One study demonstrates a functional difference between alternate leaf shapes in a heterophyllic violet. The other illustrates the use of phenotypic selection analysis to determine whether the optimum leaf phenotype differs among environments encountered by individual plants. Following the case studies, I review other published work relevant to the ecological and evolutionary significance of heterophylly, much of which has not been conducted explicitly in that context. In sum, this work suggests that the significance of longitudinal asymmetry created by heterophylly is case specific. Some instances of heterophylly may constitute adaptation to fine-grained environmental variation, and others may simply reflect inevitable effects of environmental factors on leaf development that have ultimately neutral or negative effects on individual fitness. The study of heterophylly provides common ground for physiologists, developmental biologists, ecologists, and evolutionary biologists. Recognition of this common interest and increased interaction among groups studying heterophylly would promote a more systematic and synthetic picture of the biological bases and significance of this widespread phenomenon.