GEOGRAPHIC MOSAICS OF EVOLVED ADAPTATIONS: A TEST OF THE HYPOTHESIS OF COEVOLUTIONARY ALTERNATION IN THE MODERN NATICID GASTROPOD PREDATOR-PREY SYSTEM

The hypothesis of coevolutionary alternation (Thompson 2005) calls for a reappraisal of how interacting predators and multiple prey species evolve. Selection is hypothesized to favor predators that preferentially consume the least defended prey species while concurrently favoring the evolution of heightened defenses in preferred prey. Prey experiencing little predation are expected to lose any previously evolved defenses. The relative levels of defense expressed by prey species is thus expected to fluctuate, with selection favoring the predator that prefers the least defended prey species at any time.

To test this hypothesis, bivalve prey defenses (shell thickness) and naticid gastropod predator preferences indicated by drilling frequencies (DF) were examined in populations along ~800 km of the U.S. east coast. Bulk samples from 18 beaches were categorized into five zones based on natural geographic breaks. In all five zones Spisula solidissima experienced the greatest DF including one zone in which it was more defended than Anadara ovalis. Our data do not support Thompson’s predictions of alternation. Naticid preferences did not vary geographically and the most highly defended prey populations experienced predation, indicating the least defended species was not preferred.

This hypothesis assumes that prey taxa have equal capacity to evolve defenses, which does not seem to be the case for this system. With a slight modification of the predictions for alternation so the strength of the preference hierarchy is expected to vary by zone and to correspond to comparative defense levels of the prey, our results support alternation. Our predictions are still valid to test for alternation because variation in relative defenses and predation intensity reveals that the selective regime for the predator varies geographically. This pattern has not been mentioned by previous studies and was not documented by a similar preliminary test (Dietl and Kelley 2007). Our conclusion is significant because coevolutionary alternation potentially explains how species might continue to interact and coevolve over long periods of time with no net evolutionary trends.