View - Kowalewski, M. - Virginia Tech

DIETL AND KELLEY—PREDATOR-PREY ARMS RACESpredator-prey interaction. These systems do,however, provide more definitive results becausetraces of predation are preservable in the fossilrecord. Crab-gastropod predator-prey systemstypically have been characterized as exhibitingcoevolution (Trussell, 2000; West et al., 1991), butalternative interpretations can be offered (seeabove); the same can be said for the cassidgastropod–sea urchin predator-prey interaction. Insuch cases, the question remains open as to whatprocess was important.Our discussion of coevolution models hasrepeatedly highlighted that approach’s main pointof contention with the escalation hypothesis; thatis, predators are not expected to respond to changesin their prey (the “decoupling” argument).Coevolution models (Abrams, 1986, 1991; Kitchell,1986, 1990) have assumed that positive feedbackbetween interacting populations, in terms of changesin population size or density, or energy intake, isthe same as reciprocal adaptation (Vermeij, 1994).But are changes in population dynamics or energyintake of interacting species an appropriaterepresentative of selection-based processes (see alsoVermeij, 1994)? If progress is going to be made inthe debate about coevolution and escalation wemust search for empirical evidence of evolutionaryresponses in nature. This requires not only adescription of the products of selection in terms ofbirths and deaths, or energy intake, but also anevaluation of how interactions among organismsaffect the opportunity for adaptation (Vermeij,1994). Interactions have consequences in the formof success and failure; traces of predation allowthe ranking of importance of various selectiveagents (Vermeij, 1987). “Those agencies that affecta large number of individuals … should … play alarger role in adaptive evolution than do agenciesthat affect a minority of individuals” (Vermeij,1987, p. 23). Distinction between the two processesin the fossil record will require documentation ofsources, frequencies, and cost-benefit effects ofselection (which for many systems requiresevidence from living animals; Vermeij, 1994).Our second theme is that rates of evolutionarychange of traits important in species interactionsmay vary from very short, rapid changes taking placeon the ecological time scale (tens to hundreds ofyears; Thompson, 1998) to longer spans ofevolutionary time (millions of years; Vermeij, 1987,1992). Trends over evolutionary time are also notnecessarily directional—although directionaltrends in antipredatory characters do occur,including increases in shell thickness (Kelley andHansen, 2001) or ornamentation (Dietl et al.,2000)—but may be highly dynamic in nature(Kitchell, 1990). These findings suggest that a lackof the predicted ideal unbounded, progressive trendis not sufficient evidence to argue against theimportance of interactions among species as adriving force in evolution (Kitchell, 1990). Despitethe apparent antiquity of many predator-preyassociations in the fossil record (Boucot, 1990),there are still few detailed studies testing arms racehypotheses. This does not imply that the inherentdirectionality (at the level of “megatrajectories”; seeKnoll and Bambach, 2001) in evolution isunimportant. Given that economic inequalitiesabound in nature between enemies (Vermeij, 1999),such direction is inevitable. Such directionality doesnot assume that within evolving lineages adaptationis boundless as predicted by the Red QueenHypothesis. There are likely to be periods ofdirectionality in an arms race (increasing meanvalues of traits) but also occasional reversals or evenperiods of stasis (Dawkins, 1986; Kitchell, 1990).Vermeij (1994) also espoused this view of theepisodic nature of selection.The fossil record is the only place where thelong-term effects of interactions among species canbe traced; thus it remains a valuable resource fortesting predictions based on these processes. If thegeographic mosaic process envisioned byThompson is a common feature of the evolutionof predator-prey interactions, then escalation andcoevolution studies may have to incorporate anunderstanding of the population structure of thespecies being studied. A geographic perspective inour approach to species interactions and theprocesses driving their evolution may allow for amore diverse array of testable hypotheses on howpredator-prey systems evolve (see also Thompson,369