Geographical parthenogenesis: Opportunities for asexuality

Abstract

Asexual organisms often occupy larger and more northern distribution areas than their sexual relatives. These phenomena, summarized under the term geographical parthenogenesis, seem to confirm a short term advantage of asexual reproduction. Geographical parthenogenesis may be explained by better colonizing abilities of asexual organisms, or by a swamping of sexual populations because of introgression of asexuality. Asexual organisms may perform better in diverse and narrow ecological niches, or may benefit in colder climates from a lower pressure of parasites and predators. The distributional success of asexuals has been also referred to indirect advantages of hybridity and/or polyploidy. Sexual hybrids or polyploids, however, do not show patterns of geographical parthenogenesis. Here, I present a novel model for those asexual organisms that have originated from hybridization. Climatic changes may have triggered interspecific hybridization, which increases frequencies of new origins of asexuality, but decreases fitness of sexual progenitor species. Asexuality is further advantageous for re-colonization of devastated areas. Therefore, frequencies of asexual populations increase relative to those of related sexuals. Glaciations during the Pleistocene may have provided great opportunities for the evolution of asexual organisms, while the Tertiary might have been a period of predominant sexuality. The geologically rather recent wave of asexuality helps to explain that most extant asexual animals and plants are evolutionarily young and appear scattered on the tips of phylogenetic trees.