While sexual selection is known to be a major contributor to speciation in animals, its role in plant speciation is largely unknown. However, the conditions for sexual selection are exceptionally favorable in plants because the number of pollen grains deposited on stigmas often greatly exceeds that needed to fertilize ovules. In this presentation, I will show that the strength of sexual selection, as estimated by the number of ovules per ovary, contributes to variation across flowering plants in (1) the compatibility of interspecific crosses; (2) rates of molecular evolution in genes with pollen-specific expression patterns; (3) species richness. In addition, I discuss the possibility that cryptic reinforcement frequently arises in plants as a consequence of good genes sexual selection, and describe a genomic signature of reinforcement that might be useful in establishing the frequency of this evolutionary mechanism. These results challenge conventional views of how plant species arise and are likely to modify research priorities in this discipline.