Asexual Speciation

Abstract

Population and evolutionary genetic theory shows that asexual organisms can undergo speciation to form inclusive populations that are independent arenas for the evolutionary processes of mutation, selection, and random drift; in other words, evolutionary species. Asexual speciation occurs when members of a species become physically isolated from each other for a long time or undergo divergent selection for adaptation to different niches. Such species form genotypic clusters separated by long-lasting gaps, as opposed to transient gaps due to random genetic drift. Species will often be phenotypic clusters as well, but these clusters may be cryptic; such cryptic species will be detectable only by genetic means. Our theoretical model of the nature and origin of species suggests two different methods of assigning individuals to species using gene sequence data. One combines population genetics and sampling theory to identify clusters that are separated by gaps too deep to be caused by drift. The other method uses the change in branching rates of lineages in a phylogenetic tree to detect the transition from between-species to within-species branching. These species criteria have been used to demonstrate that bdelloid rotifers and oribatid mites have undergone speciation; the two criteria show a reassuring amount of agreement in delimiting species. Not surprisingly, some of the resulting species are cryptic, not distinguishable by phenotype. At least some of these asexual species are adapted to different ecological niches. Our species concept and criteria can be used to test theories about the population genetics and evolutionary diversification of asexual organisms.