MATING-SYSTEM EVOLUTION IN FLOWERING PLANTS - MICROEVOLUTIONARY AND MACROEVOLUTIONARY APPROACHES

The hermaphroditism of most plants and their reliance on vectors for p ollen dispersal complicates mating patterns in comparison with most an imal groups. Since Darwin's early work there has been considerable int erest in the causes and consequences of self- and cross-fertilization in plant populations. Most research on this topic has been ahistorical in perspective and conducted almost exclusively within a selectionist framework. It is suggested that a broadened view encompassing both mi cro- and macroevolutionary analysis is necessary for a comprehensive u nderstanding of how and why mating systems evolve from one state to an other. An attempt to illustrate the diverse approaches that can be use d in studying mating-system evolution in flowering plants is presented using the heterostylous, aquatic genus Eichhornia as a model system. Evidence from molecular phylogenetic reconstruction, large-scale surve ys of geographical variation and manipulations of experimental populat ions are used to address the issue of how often the shift from outcros sing to selfing has occurred and what evolutionary mechanisms are invo lved. Results indicate that selfing has originated on several occasion s in the genus and that interactions between genetic drift and natural selection cause destabilization of heterostyly and the evolution of p redominant self-fertilization. It is argued that geographical patterns of intraspecific variation in reproductive traits are likely to provi de the critical link between micro- and macroevolutionary approaches t o the study of plant mating systems.