Nutrients, temperature, disturbance, and evolution: a model for the late Cenozoic marine record of the western Atlantic

Major changes in the marine biota of the western Atlantic region occurred over the last five million years, but the causes of these changes, and especially the relative roles of changes in temperature and nutrients in affecting them, have been controversial. The resolution of this issue has implications beyond this particular time and region because they include two environmental perturbations (the formation of the Central American Isthmus and the initiation of northern hemisphere glaciation) of global significance. Analysis of the western Atlantic late Neogene may also offer insights of broader significance into how environmental disturbance affects evolution, especially the interaction between the processes of extinction and speciation. Review of available data on paleoenvironments and biotas (including mollusks, corals, foraminifera, ostracodes, marine mammals, sea birds, and sea grasses) within the context of an explicit theoretical framework for the process of allopatric speciation allows construction of a model for evolution in the region during this time. The framework breaks speciation down into formation, persistence, and differentiation of isolated populations. The model for the western Atlantic proposes that many of the environmental and changes observed in the record of this region over the past five million years can be connected via this framework, especially around the role of environmental disturbance; disturbance connects extinction and speciation because the event of speciation may lie on a continuum of disturbance between population fluctuation and extinction. Although changes in temperature (and perhaps other factors) may have had significant effects, it was change in nutrient conditions — which most likely created conditions of habitat disturbance conducive to both enhanced speciation and extinction — that played the dominant role in causing the observed patterns of origination and extinction in the Plio-Pleistocene of the western Atlantic.