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Kurzban, 2006). In psychology and neuroscience, it is common to think of brain mechanisms as falling into two categories: specialized and general-purpose. Specialized mechanisms are frequently associated with the idea of cognitive “modules,” which are in turn associated with several kinds of property (Fodor, 1983). Modules are often held to be “innate” in the sense that they develop similarly or identically across individuals, regardless of environmental input (i.e., they are canalized). They are “domain-specific,” that is, tailored to specific tasks or types of information. In addition, they operate autonomously or “automatically,” that is, independently of other systems and processes, including consciousness, and therefore produce the same outcomes regardless of context. Nonmodular processes, on the contrary, are held to be domain-general, developmentally plastic instead of innate, and interactive rather than autonomous. Many psychologists believe that human cognition can be accounted for by some mix of these two types of mechanism. This is sometimes called a “dual systems” view (Stanovich, 2004).

This view, derived from models of perception, equates specialization only with highly local, narrow, and stereotyped processes, and defines general-purpose processes as whatever “modular” processes are not (Fodor, 1983). Empirically, this means that evidence for developmental plasticity, interactivity, or capacity to respond to evolutionarily novel stimuli is typically taken as evidence that a brain region or process is not evolutionarily specialized. Moreover, proximal factors such as plasticity and developmental constraint are sometimes seen as alternatives to explanations invoking selection for particular outcomes (Elman et al., 1996). Biologically speaking, however, these distinctions may be based on false dichotomies. There is no reason why adaptations in the brain (or elsewhere) need to be developmentally canalized as opposed to plastic, isolated from other systems rather than interactive, or tightly locked to specific categories of information regardless of developmental circumstance.

If adaptations in the brain resemble other organismal adaptations—for example, tissue types, limbs, organs, and the molecular machinery of cells—they are likely to be both heterogeneous and hierarchical. Heterogeneity arises from the fact of form-function fit: adaptations have different histories and have evolved to do different things, so they are likely to have diverse properties rather than coming in just two kinds. Hierarchical organization, in turn, is characteristic of systems that evolve via descent with modification. Because new structures evolve from older structures, adaptations frequently share a mix of ancestral and derived features, with relatively ancient features (e.g., properties of neurons in general) shared more widely across organismal structures, and relatively recent ones (e.g., properties of specialized brain regions) more narrowly distributed, in a hierarchically organized fashion (Carroll et al., 2005).

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