Pelagic species diversity, biogeography, and evolution

Pelagic (open-ocean) species have enormous population sizes and broad, even global, distributions. These characteristics should damp rates of speciati on in allopatric and vicariant evolutionary models since dispersal should s wamp diverging populations and prevent divergence. Yet the fossil record su ggests that rates of evolutionary turnover in pelagic organisms are often q uite rapid, comparable to rates observed in much more highly fragmented ter restrial and shallow-marine environments. Furthermore, genetic and ecologic al studies increasingly suggest that species diversity is considerably high er in the pelagic realm than inferred from many morphological taxonomies. Zoogeographic evidence suggests that ranges of many pelagic groups are much more limited by their ability to maintain viable populations than by any i nability to disperse past tectonic and hydrographic barriers to population exchange. Freely dispersing pelagic taxa resemble airborne spores or wind-d ispersed seeds that can drift almost anywhere but complete the entire life cycle only in favorable habitats. It seems likely that vicariant and allopa tric models for speciation are far less important in pelagic evolution than sympatric or parapatric speciation in which dispersal is not limiting. Nev ertheless, speciation can be quite rapid and involve cladogenesis even in c ases where morphological data suggest gradual species transitions. Indeed, recent paleoecological and molecular studies increasingly suggest that clas sic examples of "phyletic gradualism" involve multiple, cryptic speciation events. Paleoceanographic and climatic change seem to influence rates of turnover b y modifying surface water masses and environmental gradients between them t o create new habitats rather than by preventing dispersal. Changes in the v ertical structure and seasonality of water masses may be particularly impor tant since these can lead to changes in the depth and timing of reproductio n. Long-distance dispersal may actually promote evolution by regularly carr ying variants of a species across major oceanic fronts and exposing them to very different selection pressures than occur in their home range. High di spersal in pelagic taxa also implies that extinction should be difficult to achieve except though global perturbations that prevent populations from r eestablishing themselves following local extinction. High rates of extincti on in some pelagic groups suggests either that global perturbations are com mon, or that the species are much more narrowly adapted than we would infer from current taxonomies.