The Evolutionary Maintenance of Sexual Reproduction: Evidence from the Ecological Distribution of Asexual Reproduction in Clonal Plants

In theory, females that reproduce asexually should enjoy a twofold advantage in fitness over sexual females, yet sex remains the predominant mode of reproduction in virtually all eukaryotes. The evolutionary maintenance of sex is especially puzzling in clonal plants because the transition from sexual to exclusively asexual reproduction is an ever-present possibility in these species. In this article, I use published data on the genotypic diversity of populations of clonal plants to test five hypotheses about the ecological situations that limit or favor clonal reproduction in vascular plants. The data were drawn from 248 studies covering 69,000 individuals in >2000 populations of 218 species in 74 plant families. The tests showed the following: (1) the frequency of clonality increases with population age, indicating that clonal reproduction is limited by disturbance; (2) clonal reproduction is limited by dispersal; clones are more frequent in aquatic and apomictic species in which the dispersal of clonally produced propagules is less limiting; (3) clones are more frequent in populations of rare or endangered species; (4) populations of alien plants have higher frequencies of clonality; and (5) clones are more frequent at the edges of species’ geographical ranges. Thus, it appears that the ultimately successful clonal plant would be a rare, aquatic, alien apomict living in an undisturbed, geographically marginal habitat. Since this combination of circumstances is so restrictive, it is perhaps better regarded as a sign of sexual failure than as a recipe for clonal success.