View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002of a positive-feedback system of coevolving bioticinteractions, and the changes may be sudden anddiscontinuous as well as gradual and continuous(Kitchell 1990). This view contrasted markedlywith Futuyma and Slatkin’s (1983b) conclusionthat the “ideal paleontological evidence [forcoevolving lineages] would be a continuous depositof strata in which each of two species showsgradual change in characters that reflect theirinteraction.” In this restricted view of Futuyma andSlatkin, if prey shell thickness is a deterrent topredation, increased predation pressure shouldfavor the evolution of thicker shells in the prey.Similarly, if the predator increases in size (invadingthe size refuge of the prey), general models predictan increase in prey size as the likely evolutionaryresponse. The model results summarized inKitchell (1990) suggest that these examples are notthat straightforward, and general theory can predictprey responses other than increased size or shellthickness, even without any change in the directionof selection. Thus the lack of the intuitivelyexpected “linear” response in arms races betweenpredator and prey cannot be used as evidence thatbiotic interactions were unimportant in evolution.When is it coevolution and not escalation?—Although the analogy of an arms race incoevolution between predators and prey has beenwidely used (and assumed) to characterizepredator-prey interactions, such studies often lackempirical evidence that the predator respondsevolutionarily to its prey (see Brodie and Brodie,1999). Despite the assumption of a tight reciprocallink between naticid gastropods and their bivalveprey in the models of DeAngelis et al. (1984, 1985),there is no empirical evidence of reciprocaladaptation. Kelley (1989) interpreted the changesin thickness of the Miocene bivalves she studiedas a response to naticid predation; species preyedupon most heavily displayed the greatest increasein thickness. However, evolutionary changes in thepredator are more interpretable as defenses againstthe gastropod predator’s own enemies. Kelley(1992) did not find any significant trends in mostcharacters thought to affect predator efficiency. Onecharacter of the predator that could be interpretedas a reciprocal response to evolution in the prey wasa size increase in the predatory genus Neverita (sizewas an important morphological character in themodels of coevolution discussed previously). Kelley(1992) pointed out, however, that the size increasein Neverita also could be interpreted as anevolutionary response to its own enemies. Thisinterpretation is supported by the fact that thepredator’s shell thickness also increased, most likelyas a response to the naticid’s own predators.The study of the interaction between crabs andgastropods in Lake Tanganyika, Africa, alsohighlights the lack of empirical evidence for predatorresponse to prey. In general, Tanganyikan gastropodshave thicker, more ornate shells (unusual forfreshwater species; Vermeij and Covich, 1978) thanother closely related lacustrine taxa from outside thelake. They also display a considerably higherincidence of shell repair (a measure of theeffectiveness of the shell as a defense) in responseto unsuccessful predatory attacks by crabs (West etal., 1991). The endemic Tanganyikan crabs possesslarger, more robust crushing claws than other Africancrabs. A reciprocal coevolutionary arms race wasinvoked to explain the observed pattern (West et al.,1991, p. 605): “To protect themselves fromdurophagous predators, Tanganyikan gastropodshave increased their shell size, strength, andsculpture. Tanganyikan crabs have concordantlyincreased their shell-crushing capacity with largerobust chelae lined with broad molariformdentition”—it is the argument that claws getstronger, so shells get thicker, so claws get strongerstill. While it is clear that prey shell charactersevolved in response to selection from predators, acorrelation between the morphological features ofthe predator and prey does not unequivocallyestablish reciprocal selection and evolutionbetween predator and prey. It is equally probablethat the prey responded evolutionarily to the crabs,but the crabs, instead of responding to their prey,evolved in response to competition with other crabsfor prey, space, and/or mates (an escalationinterpretation; Vermeij, 1978).McNamara and Long (1998) implicated acoevolutionary arms race in the evolutionary trends358