Adaptations, By-products, and Random Effects
The most fundamental analytic tool for recognizing an adaptation is its definition. Stripped of complications and qualifications, an adaptation is ( 1) a system of inherited and reliably developing properties that recurs among members of a species that (2) became incorporated into the species’ standard design because during the period of their incorporation, (3) they were coordinated with a set of statistically recurrent structural properties outside the adaptation (either in the environment or in the other parts of the organism), (4) in such away that the causal interaction of the two (in the context of the rest of the properties of the organism) produced functional outcomes that were ultimately tributary to propagation with sufficient frequency (i.e., it solved an adaptive problem for the organism). (For a more extensive definition of the concept of adaptation, see Tooby & Cosmides, 1990b). Adaptations are mechanisms or systems of properties crafted by natural selection to solve the specific problems posed by the regularities of the physical, chemical, developmental, ecological, demographic, social, and informational environments encountered by ancestral populations during the course of a species’ or population’s evolution (for other discussions of adaptation, see Pinker & Bloom, this volume; Symons, 1989, this volume; R. Thornhill, 1991; Tooby & Cosmides, 1990a; Williams, 1966, 1985; see Dawkins, 1986, for his discussion of adaptations under the name adaptive complexity).
Thus, chance and selection, the two components of the evolutionary process, explain different types of design properties in organisms, and all aspects of design must be attributed to one of these two forces. Complex functional organization is the product and signature of selection. Reciprocally, the species-typical properties of organisms attributable to chance will be no more important, organized, or functional than can be attributed to chance. The conspicuously distinctive cumulative impacts of chance and selection allow the development of rigorous standards of evidence for recognizing and establishing the existence of adaptations and distinguishing them from the non- adaptive aspects of organisms caused by the nonselectionist mechanisms of evolutionary change (Pinker & Bloom, this volume; Symons, this volume; R. Thornhill, 1991 ; Tooby & Cosmides, 1990b; Williams, 1966, 1985). Complex adaptations are usually species-typical (Tooby & Cosmides, 1990a); moreover, they are so well-organized and such good engineering solutions to adaptive problems that a chance coordination between problem and solution is effectively ruled out as a plausible explanation. Adaptations are recognizable by “evidence of special design " (Williams, 1966); that is, by recognizing certain features of the evolved species-typical design of an organism ''as components of some special problem-solving machinery” (Williams, 1985, p. 1) that solve an evolutionarily long-standing problem. Standards for recognizing special design include factors such as economy, efficiency, complexity, precision, specialization, and reliability, which, like a key fitting a lock, render the design too good a solution to a defined adaptive problem to be coincidence (Williams, 1966). Like most other methods of empirical hypothesis testing, the demonstration that something is an adaptation is always, at the core, a probability assessment concerning how likely a situation is to have arisen by chance. The lens, pupil, iris, optic nerve, retina, visual cortex, and so on, are too well coordinated both with each other and with environmental factors-such as the properties of light and the reflecting properties of surfaces-to have arisen by chance.
In addition to adaptations, the evolutionary process commonly produces two other outcomes visible in the designs of organisms: ( 1) concomitants or by-products of adaptations (recently nicknamed “spandrels”; Gould & Lewontin, 1979); and (2) random effects. The design features that comprise adaptations became incorporated into the standard design because they promoted their own frequency and are, there- fore, recognizable by their organized and functional relationships to the rest of the design and to the structure of the world. In contrast, concomitants of adaptations are those properties of the phenotype that do not contribute to functional design per se, but that happen to be coupled to properties that are, and so were dragged along into the organism’s design because of selection on the design features to which they are linked. They may appear organized, but they are not functionally organized.
The explanation for any specific concomitant or spandrel is, therefore, the identification of the adaptation or adaptations to which it is coupled, together with the reason why it is coupled. For example, bones are adaptations, but the fact that they are white is an incidental by-product. Bones were selected to include calcium because it conferred hardness and rigidity to the structure (and was dietarily available), and it simply happens that alkaline earth metals appear white in many compounds, including the insoluble calcium salts that are a constituent of bone. From the point of view of functional design, by-products are the result of “chance,” in the sense that nothing in the process of how they came to be incorporated into a design other than sheer coincidence would cause them to be coordinated solutions to any adaptive problem. For this reason, by-products are expected not to contribute to the solution of adaptive problems more often or more effectively than chance could explain.