Hydrographic and Tectonic Control of Plankton Distribution and Evolution

Abstract

The enormous population size and cosmopolitan distributions of many pelagic species in everything from protists to vertebrates suggest that many of them should be relatively resistant to speciation and extinction. Yet evolutionary rates in many pelagic taxa are fast compared to shallow marine benthos whose populations and geographic distributions are smaller by several orders of magnitude. Genetic and zoogeographic analysis suggest that dispersal is relatively easy for many groups of pelagic fish, pteropods, salps, and planktic foraminifera, compounding the apparent problem of the relatively high rates of evolutionary turnover in many pelagic taxa. Nonetheless, the geographic distributions of these pelagic groups are closely correlated with surface watermasses suggesting that although individuals can cross major hydrographic boundaries, they can grow and reproduce within a much more restricted variety of habitats. In this respect, freely dispersing pelagic taxa resemble wind-dispersed seeds that can go almost anywhere but grow only in favorable environments. The habitat specificity of pelagic groups rather than an inability to disperse across hydrographic fronts may make them vulnerable to major paleoceanographic change and account for their relatively rapid rates of evolutionary turnover. High dispersal capability of these organisms suggests that geographic distance, and vicariant subdivision of populations alone are not an effective means of producing new species unlike the pattern in many terrestrial clades. Tectonic barriers to dispersal between ocean basins apparently do not create new species of pelagic taxa by limiting dispersal. Instead, tectonism and climate change contribute to speciation by changing the character of surface watermasses and creating habitats inwhich newly evolved taxa can thrive. High dispersal ability also means that many pelagic groups cannot be driven to extinction by gradual habitat modification since even populations in small refugia have the potential to rapidly reestablish global populations.