Animal behavior is subject to the action of natural selection, favoring individuals that behave in ways that maximize their fitness by promoting individual survival and reproductive success. Cultural evolution plays an important role, with behavioral traits of surprising complexity spreading rapidly through a population. Behavioral ecologists measure the costs and benefits of alternative types of behavior to gain an understanding of basic behavioral processes such as territory defense, foraging, and mating. This cost–benefit approach allows quantitative predictions of behavior tightly tied to fitness, such as how long to guard a mate, or how long to forage at a particular location before moving on. Both physiological factors, such as the need to keep eggs warm, and ecological factors, such as the spatial distribution of resources, can influence the evolution of mating systems. In many species, individuals cooperate with each other in procuring food or defending against predators. Hamilton’s model of indirect selection is one possible explanation for the evolution of behavior favoring relatives, including the astounding degree of cooperation in eusocial animals. But in some species, cooperation is common even among unrelated individuals; in these cases game theory models may help explain the evolution of cooperation.

● Two types of adaptationism are distinguished – ontological and methodological. The ontological thesis asserts that natural selection is the only important cause of the evolution of the vast majority of phenotypic traits; the methodological thesis tells scientists to construct idealized models of the evolution of phenotypic traits that ignore genetics and assume that organisms reproduce asexually. ● The ontological thesis fails to be true of a trait’s evolution if the genetic system prevents the fittest of the available traits from evolving to fixation and if the character states of present organisms are strongly influenced by the character states of their ancestors. ● An optimality model for a trait is important in evolutionary biology, even if the individuals in the population do not have the optimal trait value. ● Since science often progresses when different investigators adopt different approaches to the same problem, evolutionary biology may benefit by having some investigators be methodological adaptationists even if it would be bad for the science if all investigators did so.