The evolution of alternative strategies and tactics

Summary Discrete variation where one sex has more than one means of achieving the same functional end, such as mating or nesting (alternative strategies and tactics), requires an evolutionary explanation. Discrete differences in behavior, morphology, color, and life-history patterns are found in many contexts, such as alternative reproductive patterns, social insect castes, sex allocation, and mimicry polymorphisms. In this chapter I draw on the literature of these fields to identify similar patterns and common models that can be applied to our understanding of alternative phenotypes in general. I emphasize the invertebrate literature throughout because excellent reviews are available for vertebrates. Traditionally, the behavioral literature has separated alternative phenotypes into those due to genetic differences (e.g., polymorphisms) and those due to environmental or individual cues (e.g., conditional tactics). Such categories are impractical and they force us to dichotomize behavior as due to nature or nurture. Drawing on the well-developed sex-allocation literature, I suggest a different system for categorizing alternative phenotypes: irreversible or adult lifetime patterns (e.g., dioecy) and reversible or facultative patterns (e.g., hermaphroditism), which can be further subdivided into sequential and simultaneous. This new system for organizing alternative phenotypes allows us to see that condition dependence and frequency dependence may occur under any category. In general, alternative phenotypes can be maintained in a population when their fitness curves cross, that is, when each does better than the other under some conditions. A variety of factors can cause fitness curves to cross. Negative frequency dependence is one, which has the added advantage of explaining how patterns can be maintained at a stable frequency. Probably the most common explanation for the maintenance of discrete variants is switches in phenotype based on environmental or individual, condition-based cues. What are the circumstances that favor the evolution of irreversible phenotypes over switching? Lifetime patterns are favored when individuals that switch have lower fitness than those that do not. Lifetime patterns allow complex suites of adaptations to develop for each phenotype that are coordinated through physiological and developmental processes. When environmental conditions are predictable and correlate with fitness, selection favors the animal switching from one pattern to the other at the time that maximizes fitness. However, when the environmental conditions that favor two phenotypes are unpredictable and change frequently, selection favors animals that invest in what is necessary to perform in either role. Under most circumstances the animal will use decision rules based on information about what maximizes fitness, but if the individual does not have such information, yet must choose between exclusive alternatives, then selection will favor an arbitrary decision rule. A promising approach to studying alternative phenotypes is dynamic, state-variable modeling ( Houston and McNamara, 1999 ; Clark and Mangel, 2000 ), which is particularly good for evaluating the interacting effects of the organism's state, of environmental and social conditions, and of density and frequency dependence. When such multiple factors are evaluated together, surprising predictions about the conditions under which individuals should follow particular patterns sometimes result. An understanding of the evolution of alternative phenotypes requires a strong, integrative approach that seeks to understand the cues and mechanisms controlling behavioral decisions. When this approach is combined with dynamic, state-based models, it is possible to make predictions about the circumstances that favor the evolution of particular decision-making patterns over others.