Patterns of speciation in the rotifer species complex, Brachionus plicatilis

Although the past two decades have been a particularly productive and dynamic period in the study of speciation, researchers still struggle to identify general trends among the observed wide diversity of phenomena. This is largely because very few systems are amenable to the necessary comparative studies. Consequently, speciation research has focused on only a few model systems, and even fewer comparative studies, between many populations and species within a taxon, have been conducted.; The goal of the present thesis was to explore a potentially fruitful new system for speciation work, the rotifer species complex, Brachionus plicatilis. The investigation had three objectives: (1) to form a phylogenetic hypothesis for the group, (2) to develop a species boundaries hypothesis from phylogenetic and reproductive data, and (3) to perform a comparative study between diverged populations within the B. plicatilis complex to determine whether there is a predictable sequence in which different reproductive barriers emerge.; Phylogenetic reconstruction indicates that B. plicatilis is an ancient cryptic complex comprised of at least 15 ancient lineages likely to have species status. Pairwise sequence divergences indicate that B. plicatilis underwent a single radiation, perhaps 10--20 million years ago, with little evidence of speciation since. Efficient passive dispersal has since given many of the species a global distribution, and coexistence of widely genetically divergent species is common.; Crosses between a range of genetically diverged populations within B. plicatilis s.s. suggest the following sequence of character evolution in the evolution of reproductive isolation. Reproductive isolation evolves gradually, with an initial decline in the fertilization system and a subsequent decline in the mate recognition system. Together, prezygotic mechanisms of isolation are sufficient to cause reproductive isolation between genetically distant lineages. There is little evidence that postzygotic barriers play a large role in the evolution of reproductive isolation in this group.