Molecular genetic analyses of species boundaries in the sea

The tools of molecular genetics have enormous potential for clarifying the nature and age of species boundaries in marine organisms. Below I summarize the genetic implications of various species concepts, and review the resul ts of recent molecular genetic analyses of species boundaries in marine mic robes, plants, invertebrates and vertebrates. Excessive lumping, rather tha n excessive splitting, characterizes the current systematic situation in ma ny groups. Morphologically similar species are often quite distinct genetic ally, suggesting that conservative systematic traditions or morphological s tasis may be involved. Some reproductively isolated taxa exhibit only small levels of genetic differentiation, however. In these cases, large populati on sizes, slow rates of molecular evolution, and relatively recent origins may contribute to the difficulty in finding fixed genetic markers associate d with barriers to gene exchange. The extent to which hybridization blurs s pecies boundaries of marine organisms remains a subject of real disagreemen t in some groups (e.g. corals). The ages of recently diverged species are l argely unknown; many appear to be older than 3 million years, but snails an d fishes provide several examples of more recent divergences. Increasingly sophisticated genetic analyses make it easier to distinguish allopatric tax a, but criteria for recognition at the species level are highly inconsisten t across studies. Future molecular genetic analyses should help to resolve many of these issues, particularly if coupled with other biological and pal eontological approaches.