Attention to mechanisms invites a concrete analysis of the nature-nurture question: how should a behavioral ecolo-gist view the relationship between genetic and environmental influence on behavior? As already indicated, a behavioral phenotype, like any other aspect of the phenotype, is expressed when some input affects the structure or state of a preexisting phenotype and causes it to change. Behavior is movement produced in response to some internal or external stimulus. The stimulus affects a sensory-response structure that has been formed under the influence of many environmental and genetic factors during individual development. So expression of a particular behavior, and its variation within a population, is inevitably influenced by both genotype and environments during the individual's developmental history and beyond, for individual development begins with an already-structured phenotype in the form of an egg whose organization and content depend in part on the maternal environment. As further discussed below, behavior-specific gene expression allows genetic variation in behavior to respond to selection, permitting the adaptive evolution of behavior just as of any other aspect of the phenotype.
In one of the few discussions of classical ideas regarding phenotypic plasticity written by a behavioral ecologist, Sih noted that even though the usual definition of phenotypic plasticity would cover all aspects of the phenotype, including behavior, in fact ''behaviors, despite their obvious plasticity, are often explicitly excluded from the rubric of phenotypic plasticity.'' Sih goes on to distinguish between behavioral and 'developmental' plasticity, which he defines as plasticity in morphology and life-history traits, in order to emphasize how studies of adaptation in behavioral ecology can broaden understanding of plasticity in general. While this relatively narrow definition of developmental plasticity is suitable and clear for his purposes, the more inclusive definition of developmental plasticity given here underscores the fact that developmental changes underlie all phenotypic plasticity. An inclusive approach is important because so many of the same patterns and principles relating to developmental plasticity in general underlie the structure and evolution of behavioral phenotypes. Such principles are emphasized in the rest of this article, highlighting their applications to behavior.
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