If an ancestral stem group repeatedly colonizes similar environments, developmental plasticity specific to that group should consistently give rise to similar phenotypes. Parallel selection on those similar phenotypes could lead to the repeated evolution of characteristic ecotypes, a property common to many adaptive radiations. A key prediction of this "flexible stem" model of adaptive radiation is that patterns of phenotypic divergence in derived groups should mirror patterns of developmental plasticity in their common ancestor. The threespine stickleback radiation provides an excellent opportunity to test this prediction because the marine form is representative of the ancestral stem group, which has repeatedly given rise to several characteristic ecotypes. We examined plasticity of several aspects of shape and trophic morphology in response to diets characteristic of either the derived benthic ecotype or the limnetic ecotype. When marine fish were reared on alternative diets, plasticity of head and mouth shape paralleled phenotypic divergence between the derived ecotypes, supporting the flexible stem model. Benthic and limnetic fish exhibited patterns of plasticity similar to those of the marine population; however, some differences in population means were present, as well as subtle differences in shape plasticity in the benthic population, indicating a role for genetic accommodation in this system.

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Changing environments, whether through natural or anthropogenic causes, can lead to the loss of some selective pressures (‘relaxed selection’) and possibly even the reinstatement of selective agents not encountered for many generations (‘reversed selection’). We examined the outcome of relaxed and reversed selection in the adaptive radiation of the threespine stickleback fish, Gasterostues aculeatus L., in which isolated populations encounter a variety of predation regimes. Oceanic stickleback, which represent the ancestral founders of the freshwater radiation, encounter many piscivorous fish. Derived, freshwater populations, on the other hand, vary with respect to the presence of predators. Some populations encounter native salmonids, whereas others have not experienced predation by large fish in thousands of generations (relax-selected populations). Some relax-selected populations have had sport fish, including rainbow trout, Oncorhynchus mykiss, introduced within the past several decades (reverse-selected). We examined the behavioural responses of stickleback from three populations of each type to simulated attacks by trout and birds to determine whether relaxed and reversed selection has led to divergence in behaviour, and whether this divergence was predator specific. Fish from trout-free populations showed weak responses to trout, as predicted, but these responses were similar to those of oceanic (ancestral) populations. Fish from populations that co-occur with trout, whether native or introduced, showed elevated antipredator responses, indicating that in freshwater, trout predation selects for enhanced antipredator responses, which can evolve extremely rapidly. Comparison of laboratory-reared and wild-caught individuals suggests a combination of learned and genetic components to this variation. Responses to a model bird flyover were weakly linked to predation environment, indicating that the loss of predation by trout may partially influence the evolution of responses to birds. Our results reject the hypothesis that the consistent presence of predatory birds has been sufficient to maintain responses to piscivorous fish under periods of relaxed selection.

Three focal size classes [small (<40 mm, standard length, L S ), medium (40-80 mm L S ) and large (>80 mm L S )] of pumpkinseed Lepomis gibbosus were exposed to conspecific or one of three heterospecific chemical alarm cues under one of three levels of habitat complexity (low, intermediate and high). Under low and intermediate habitat complexities, pumpkinseeds <80 mm L S exhibited a significant antipredator response when exposed to the chemical alarm cues of heterospecific prey guild members (largemouth bass Micropterus salmonides and hypoxanthine-3-N-oxide, the putative Ostariophysan alarm 'pheromone'), while pumpkinseeds >80 mm L S exhibited a foraging response. Under highly complex habitats, pumpkinseeds of all three size classes exhibited an antipredator response. Moreover, under all three habitat conditions, pumpkinseeds of all size classes exhibited an antipredator response to conspecific alarm cues. There was no change in behaviour under any treatment combination in response to an allopatric heterospecific control (swordtails Xiphophorus helleri). These results provide the first field verification of the size dependent use of chemical alarm cues by centrarchids and demonstrate that microhabitat complexity significantly influences the threat-sensitive use of these cues. # 2005 The Fisheries Society of the British Isles

When confronted by potential predators, many prey fishes engage in predator inspection behavior. Previous authors have argued that by selectively avoiding the predator's head during an inspection visit (attack cone avoidance), individual inspectors may reduce their local risk of predation. In field trials, we investigated the effects of predator diet cues on the presence of 'attack cone avoidance' during predator inspection visits. Wild, free-ranging finescale dace (Phoxinus neogaeus) were exposed to the combined cues of a model predator and a distilled water control or the odor of a yellow perch (Perca flavescens) fed dace (with alarm pheromone), swordtail (Xiphophorus helleri) (lacking Ostariophysan alarm pheromone), or perch that were food deprived for four days. Finescale dace modified their predator inspection behavior following exposure to the odor of a perch fed dace (fewer dace present, reduced frequency of inspections, and an increased per capita inspection rate) compared to those exposed to the odor of a perch fed swordtails, perch that were food deprived, or a distilled water control. In addition, dace inspected the tail region more often only when the model predator was paired with the odor of a perch fed dace. In all other treatments, dace inspected the head region of the model predator more often. These data suggest that attack cone avoidance of inspecting prey fishes may be more likely to occur in high-risk situations, such as in the presence of conspecific alarm pheromones in the diet of potential predators.