Threespine sticklebacks, small fish with a circumglobal distribution in coastal marine and freshwater of the northern hemisphere, present a remarkable scope of variation in body and fin shape among populations. The repeated evolution of divergent body shapes in a radiation of stickleback from Cook Inlet, Alaska suggests that diversification has proceeded by extensive parallel selection. To explore this hypothesis, hydromechanical equations of fish propulsion and descriptions of stickleback foraging and anti-predator behaviours were used to develop a series of hypotheses that predicted the evolutionary effects of native predatory fishes (NPF) and relative littoral area (RLA) on body shape. Body shape was measured using Cartesian coordinates of anatomical landmarks transformed by the generalized resistant fit superimposition. In general, the results were consistent with the hypotheses that (1) RLA has a direct effect on selection for foraging behaviour and morphology, (2) RLA has an indirect effect on selection for morphology employed in predator evasion, (3) presence of NPF has a direct effect on selection for evasive morphology, and (4) presence of NPF has an indirect effect on selection for foraging behaviour and morphology. The magnitude of the divergence of body shapes present in the Cook Inlet system suggests that extreme phenotypes have evolved by opportunistic expansion into new habitat relatively free of interspecific competition. 0 I997 'The Linnran Society of London ADDITIONAL

Summary1. Nearly all fish evade predation strikes by rapidly accelerating out of the strike path, a behaviour called the fast-start evasion response. The many studies investigating morphological, behavioural and ecological correlates of fast-start performance assume that faster starts increase the probability of evasion. 2. We tested this faster-start hypothesis by measuring the effect of acceleration ability on evasion outcome (success, failure) in Guppies ( Poecilia reticulata ) evading the strike of a natural predator, the Pike Cichlid ( Crenicichla alta ). 3. Four parameters affect evasion outcome: two parameters important to the predatorprey interaction but not to the faster-start hypothesis -(1) the time required to reach the prey by the striking predator (measured by the initial distance between predator and prey and strike velocity), (2) the evasion path of the prey relative to the strike path of the predator; and two parameters relevant to the faster-start hypothesis -(1) the ability of the prey to generate rapid tangential acceleration (measured by net distance travelled, maximum velocity, and maximum acceleration), and (2) the ability of the prey to rapidly rotate during the initial stage of the fast start. 4. On average, a one standard deviation increase in fast-start performance increases the odds of surviving a predation strike 2·3-fold. These results support the assumption that faster starts increase the probability of successfully evading a predation strike.

The empirical study of natural selection reveals that adaptations often involve trade-offs between competing functions. Because natural selection acts on whole organisms rather than isolated traits, adaptive evolution may be constrained by the interaction between traits that are functionally integrated. Yet, few attempts have been made to characterize how and when such constraints are manifested or whether they limit the adaptive divergence of populations. Here we examine the consequences of adaptive life-history evolution on locomotor performance in the live-bearing guppy. In response to increased predation from piscivorous fish, Trinidadian guppies evolve an increased allocation of resources toward reproduction. These populations are also under strong selection for rapid fast-start swimming performance to evade predators. Because embryo development increases a female's wet mass as she approaches parturition, an increased investment in reproductive allocation should impede fast-start performance. We find evidence for adaptive but constrained evolution of fast-start swimming performance in laboratory trials conducted on second-generation lab-reared fish. Female guppies from high-predation localities attain a faster acceleration and velocity and travel a greater distance during fast-start swimming trials. However, velocity and distance traveled decline more rapidly over the course of pregnancy in these same females, thus reducing the magnitude of divergence in swimming performance between high- and low-predation populations. This functional trade-off between reproduction and swimming performance reveals how different aspects of the phenotype are integrated and highlights the complexity of adaptation at the whole-organism level.

Aquatic flight, performed by rowing or flapping fins, wings or limbs, is a primary locomotor mechanism for many animals. We used a computer simulation to compare the mechanical performance of rowing and flapping appendages across a range of speeds. Flapping appendages proved to be more mechanically efficient than rowing appendages at all swimming speeds, suggesting that animals that frequently engage in locomotor behaviours that require energy conservation should employ a flapping stroke. The lower efficiency of rowing appendages across all speeds begs the question of why rowing occurs at all. One answer lies in the ability of rowing fins to generate more thrust than flapping fins during the power stroke. Large forces are necessary for manoeuvring behaviours such as accelerations, turning and braking, which suggests that rowing should be found in slow-swimming animals that frequently manoeuvre. The predictions of the model are supported by observed patterns of behavioural variation among rowing and flapping vertebrates.