MOVE OR CHANGE? THE PARADOX BETWEEN ECOLOGICAL AND EVOLUTIONARY RESPONSES TO CHANGING ENVIRONMENTS

Taxa have two fundamentally different ways of successfully responding to long-term environmental change: tracking their optimal environment or adapting to new environmental conditions. The first implies that taxa have already achieved an equilibrium niche that is maintained by stabilizing selection. If circumstances permit, they can maintain their optimal existence in the face of environmental change by moving to new locations. Little or no evolutionary change is predicted by this model, only geographic change and, perhaps, reorganization of community constituencies. The second implies that taxa abandon their former optimum and adapt to new environmental conditions. The mode of response is equivalent to the classic shifting adaptive peak model of evolution and predicts directional change in environmentally relevant traits. When viewed in a larger phylogenetic perspective, these two modes of response are reified into a more complex set of possible responses that include phylogenetic community assembly processes, parallel adaptation (homoplasy), clade-specific differences in probability of speciation, and clade turnover.

Data from Cenozoic mammals suggest that the long-term directional environmental change from warm Paleogene tropical forest ecosystems through temperate Neogene open ecosystems to cool Quaternary mixed ecosystems favored evolutionary responses, but environmental cycling through glacial and interglacial phases during the Quaternary favored ecological responses involving environmental tracking. This pattern is illustrated with data from the mammalian order Carnivora supplemented with examples from other taxa and with numerical modeling experiments. These data suggest that an important control on the balance between these two modes of response is whether environmental change drives the gain and loss of habitat types on a global scale (directional environmental change) or whether it cycles through a spectrum of existing habitat types on a local scale (cycling environmental change).