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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002FIGURE 2—Covariance between predator fitness and prey phenotype. Redrawn from Brodie and Brodie(1999, Figure 1, p. 560). A and B, see text. C, low but predictable fitness consequences (low slope butnarrow spread of points). D, low and unpredictable fitness consequences (low slope, wide spread).predatory whelks of the genus Busycon and theirtightly closing bivalve prey, Mercenaria, can beused to highlight the utility of the covarianceapproach to selection. Where they occursympatrically, more than 70% of adult Mercenariamortality often can be attributed to whelk predation(Chestnut, 1952; see also Kraeuter, 2001). Whelksactively use their shell lip to chip open their prey,a mode of attack that often inflicts shell damageon both predator and prey. Breakage must berepaired and this represents a diversion of energyaway from somatic growth or reproduction. Whelksthat often break their shells while attempting tofeed are faced with a potential tradeoff betweensubsequent reproduction and shell repair (Geller,1990). If feeding rate is reduced significantlyduring repair of the injury, breakage also may leadto a net energy loss (or negative energy budget)for the whelk resulting in a reduced growth rate.Breakage of the whelk’s shell may thus have severefitness consequences because it increases the timethe individual spends at a smaller size, leaving itmore vulnerable to its own predators like the stonecrab Menippe mercenaria (Magalhaes, 1948; Kent,1983). The selective consequences for predatorywhelks interacting with Mercenaria of varyingsizes are predictable (Fig. 2A) (Dietl, in review).Predators that select large bivalve prey increasethe likelihood that their shell lip will be broken inthe process of attempting to open their prey.Adaptive response of the predator to the prey ispossible in this system of interacting predators andprey, with predators expected to evolve increasedeffectiveness in opening prey (perhaps representedas a temporal decrease in the frequency of repairedfeeding-inflicted shell breakage). Traces of thepredation event are retrievable from the fossilrecord (Dietl, unpublished data); ongoing work isexamining the origin(s) of the chipping mode ofpredation and the evolutionary history of thepredator-prey interaction.Vermeij (1994) argued that the central issue inthe debate about escalation and coevolution is thenature of selection; most current models of360