Plain-Language Summary:
The Theory of Escalation, more commonly known as the Biological Arms Race, claims that one of the more important driving factors in long-term evolution is the presence of biological enemies. Animals respond to their predators by developing better defensive strategies and structures. Predators on the other hand continue to improve their own strategies. This type of interaction is preserved well in the fossil record, where one can see the traces of successful and unsuccessful predation preserved on fossilized skeletons (unsuccessful predation is of course very important; without it, there would be no evolutionary arms race!). The result of these interactions over long periods of time is the development of diffuse coevolutionary relationships between predators and prey, the dynamics of which are still poorly understood. This paper examines what happens to such a relationship when it is disrupted by a mass extinction, and one of the prey species is replaced by a very similar, but subtly different relative.
The setting is the Pliocene-Recent of Florida. This region suffered a mass extinction of molluscs and other marine invertebrates during the Pliocene, caused mainly by the oceanographic effects of the uplift of the Isthmus of Panama (approximately 3.1 mya). Gastropod predators of the family Naticidae, present in those waters, prey upon other shelled animals by drilling holes through the shells. Naticids generally have well developed behaviors for handling prey species, being very specific as to the size of prey selected, as well as the location of drilling on the prey's shell. We described this relationship between Pliocene naticids and a species of marine clam, Chione erosa. Chione erosa unfortunately went extinct at the end of the Pliocene, but was replaced by a close relative from neighboring waters, Chione cancellata. When Chione cancellata first appeared in Floridian waters, naticid predators decreased the size of prey taken, due possibly to subtle morphological differences between Chione erosa and Chione cancellata. Nevertheless, as time passed, prey size selection increased, but Chione cancellata has apparently responded by evolving significantly thicker shells (as a deterrent to successful drilling). In summary, the paper demonstrates that mass extinctions and subtle but significant morphological differences can play a critical role in our views of evolution and escalation.
Peter D. Roopnarine, Department of Invertebrate Zoology and
Geology, California Academy of Sciences, Golden Gate Park, San
Francisco, California 94118-4599, USA.
Amy Beussink, School of Renewable Natural Resources,
University of Arizona, Tucson, Arizona 85721, USA.