Life Cycles and Evolution of Clonal (Modular) Animals

Life cycles of clonal benthic animals are more complicated than those of aclonal species because there are more parameters to vary, and because an individual clone can grow in disconnected bits and pieces and widely different shapes. Consequently, the schedule of life-history events in clonal animals is as closely tied to their size as to their age. The bigger the animal the more likely it is to survive, and there is usually no limit to this advantage. Senescence is typically absent, or unmeasurable. Reproduction and recruitment of new individuals into local populations occurs by both sexually and asexually produced larvae, and by fragmentation. In general, asexual recruitment is more frequent. Larvae of clonal species are strongly philopatric whereas those of aclonal species are dispersed over considerable distances. These differences, and the high incidence of asexual reproduction, mean that parents, siblings, and clonemates may become mixed together within a small area. Under these conditions inbreeding is likely, and there is even evidence of selection for inbreeding in several clonal phyla. Nevertheless, clonal species tend to persist as long in the fossil record as do aclonal species. Thus the relative frequency of sexual reproduction among benthic animals, if not its incidence per se, would appear to have little or no macroevolutionary significance.