Mating-system evolution in flowering plants: Micro- and macroevolutionary approaches

The hermaphroditism of most plants and their reliance on vectors for pollen dispersal complicates mating patterns in comparison with most animal groups. Since Darwin's early work there has been considerable interest 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 micro- and macroevolutionary analysis is necessary for a comprehensive understanding of how and why mating systems evolve from one state to another. An attempt to illustrate the diverse approaches that can be used 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 surveys of geographical variation and manipulations of experimental populations are used to address the issue of how often the shift from outcrossing to selfing has occurred and what evolutionary mechanisms are involved. Results indicate that selfing has originated on several occasions in the genus and that interactions between genetic drift and natural selection cause destabilization of heterostyly and the evolution of predominant self-fertilization. It is argued that geographical patterns of intraspecific variation in reproductive traits are likely to provide the critical link between micro- and macroevolutionary approaches to the study of plant mating systems.