Hormonal involvement in the control of alternative behavioral phenotypes (such as sexually dimorphic behaviors) and cyclic activities (such as seasonal nest construction, egg-laying, provisioning, and defense) render behaviors and sets of associated behaviors modular and means that behaviors can be decoupled by hormonal change and expressed in new contexts (e.g., in the opposite sex, or in different individuals such as the queens and workers of social insects). Such reorganization of behavior can produce striking behavioral novelties that build upon genotypic capacities already present in a lineage rather than requiring a series of genetic changes that would be required to build such complex novelties anew.
Modular organization of the components of behavior (Figure 2) facilitates the evolution of behavior due to the ease with which components can be duplicated and reordered within individual repertoires. Advantageous individual differences in performance, sometimes the product of positive reinforcement and learning, may then be repeated with increased frequency, subjecting them to selection as a set and leading to behavioral divergence between populations or species. Modular repetitions and reorganization characterize the evolutionary diversification of behavior (Figure 3).
Control by polygenically influenced switches, which bestows modularity on behavioral traits, also means that the environmental sensitivity, or plasticity, of behavioral traits can be adjusted both upward and downward under natural selection: any genetic change that raises the threshold of a response to an environmental factor, or lowers the ability of an individual to pass it (liability to express the trait), increases the genetic control of trait expression. Conversely, lowering the threshold for expression increases the environmental sensitivity of trait expression. The sex determination switch in turtles and lizards, and the corresponding control of the expression of sex-limited (gender-specific) behavior, has been repeatedly changed between environmental and genetic control during evolution.
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