THE CHALLENGE OF PALEOECOLOGICAL STASIS - REASSESSING SOURCES OF EVOLUTIONARY STABILITY

The paleontological record of the lower and middle Paleozoic Appalachi an foreland basin demonstrates an unprecedented level of ecological an d morphological stability on geological time scales. Some 70-80% of fo ssil morphospecies within assemblages persist in similar relative abun dances in coordinated packages lasting as long as 7 million years desp ite evidence for environmental change and biotic disturbances. These i ntervals of stability are separated by much shorter periods of ecologi cal and evolutionary change. This pattern appears widespread in the fo ssil record. Existing concepts of the evolutionary process are unable to explain this uniquely paleontological observation of fauna-wide coo rdinated stasis. A principle of evolutionary stability that arises fro m the ecosystem is explored here. We propose that hierarchical ecosyst em theory, when extended to geological time scales, can explain long-t erm paleoecological stability as the result of ecosystem organization in response to high-frequency disturbance. The accompanying stability of fossil morphologies results from ''ecological locking,'' in which s election is seen as a high-rate response of populations that is hierar chically constrained by lower-rate ecological processes. When disturba nce exceeds the capacity of the system, ecological crashes remove thes e higher-level constraints, and evolution is free to proceed at high r ates of directional selection during the organization of a new stable ecological hierarchy.