ABSTRACT
The mating system is of prime importance for understanding the process of natural selection since it governs the character of genetic transmission and plays a central role in regulating the genetic structure of populations. Flowering plants display a great diversity of reproductive methods, often among closely related taxa. This chapter reviews work on the evolutionary breakdown of distyly and tristyly, respectively, in the Turnera ulmifolia complex and in the genus Eichhornia. Both taxa are primarily Neotropical in distribution and the breakdown of heterostyly is associated with the colonization of geographically marginal areas. Heterostylous genetic polymorphisms provide a rich source of material in which to test hypotheses concerned with the evolution of self-fertilization. Studies of two Neotropical heterostylous groups (Tumera ulmifolia complex and Eichhornia spp.) provide evidence that the breakdown of heterostyly to homostyly is associated with colonizing episodes and periods of low population density.
