Evolutionary change in faunas over geological periods of time can, in principle, be caused primarily by major changes in the physical environment, or be driven instead by continuously evolving interactions between different organisms, such as competition between predator and prey species. Given that the history of the Earth's environment is marked by both intervals of rapid change and intervals of little change, if changing external environment is mostly responsible for evolution, then we would expect evolutionary change in faunas also to vary in rate and to correlate to times of major change in past environments. If, on the other hand, biological interactions drive most evolution, a more constant rate of evolutionary change in faunas over time is anticipated and no clear correlation to changes in past environments would be expected.
In this study, deep-sea microfossils of radiolarians—a group of unicellular zooplankton that live in both surface and deep waters of the ocean—are used to examine the relative role of environmental change and biotic interaction in setting the pace of faunal evolution. The data for this study are drawn from previously published records of radiolarian species distributions in Neogene aged deep-sea sediments (ca. last 23 million years of earth history) from the Antarctic, or "Southern," Ocean. Neogene Southern Ocean environments are known to have undergone dramatic changes due to the development during this time interval of major ice-sheets on the nearby Antarctic continent. These environments thus provide a good setting to test the importance of environmental change on the evolution of planktonic biotas.
The results of the study show a clear correlation between rates of radiolarian faunal change and rates of major environmental change, particularly with respect to shifts in rates of environmental change that occurred during the mid-Miocene and end Miocene intervals. These environmental changes, in turn, are linked to major increases in Antarctic glaciation. Many Southern Ocean radiolarian species become extinct at each of these major environmental-change boundaries, with overall rates of extinction and origination shifting towards higher levels as well. Indicators of past environments preserved in these same sediments suggest, however, that it is not simply temperature to which these radiolarian faunas were most sensitive. Changes in the fertility of the ocean waters that they lived in, which are caused by physical processes such as wind-driven upwelling of deep waters, appear to have been more important. Changes in radiolarian faunas also correlate to shifts in the dominant types of planktonic algae that lived in the surface waters, that occurred at the same time as the end-Miocene glaciation. At this time there was a dramatic increase in the abundance of diatoms in the Southern Ocean, a group of algae that are thought to compete with radiolarians for silica, a key nutrient for both groups. Thus, although environmental change has been the pacesetter of Southern Ocean radiolarian evolution, biotic factors have played an important role as well.