Reproductive systems and evolution in vascular plants

Differences in the frequency with which offspring are produced asexually, t hrough self-fertilization and through sexual outcrossing, are a predominant influence on the genetic structure of plant populations. Selfers and asexu als have fewer genotypes within populations than outcrossers with similar a llele frequencies, and more genetic: diversity in selfers and asexuals is a result of differences among populations than in sexual outcrossers. As a r esult of reduced levels of diversity, selfers and asexuals may be less able to respond adaptively to changing environments, and because genotypes are not mixed across family lineages, their populations may accumulate deleteri ous mutations more rapidly. Such differences suggest that selfing and asexu al lineages may be evolutionarily short-lived and could explain why they of ten seem to be of recent origin. Nonetheless, the origin and maintenance of different reproductive modes must be linked to individual-level properties of survival and reproduction. Sexual outcrossers suffer from a cost of out crossing that arises because they do not contribute to self ed or asexual p rogeny, whereas selfers and asexuals may contribute to outcrossed progeny. Selfing and asexual reproduction also may allow reproduction when circumsta nces reduce opportunities for a union of gametes produced by different indi viduals, a phenomenon known as reproductive assurance. Both the cost of out crossing and reproductive assurance lead to an over-representation of selfe rs and asexuals in newly formed progeny, and unless sexual outcrossers are more likely to survive and reproduce, they eventually will be displaced fro m populations in which a selfing or asexual variant arises.