All living humans are products of the evolutionary process, the descendants of a long line of ancestors who succeeded in surviving, reproducing, and helping their genetic relatives. The evolutionary process acts as a series of filters. In each generation, onl a small subset of genes passes through the filte . The recurrent filtering process let only three things pass through—adaptations; by-products of adaptations; and noise, or random variations.
Adaptations are the primary product of the selective process. An adaptation can be defined as a "reliably developing structure in the o ganism, which, because it meshes with the recurrent structure of the world, causes the solution to an adaptive problem" (Tooby & Cosmides, 1992, p. 104). Adaptations might include a taste for sweet and fatty foods, the drive to defend one' s close relatives, and preferences for specifi mates, such as those that are healthy .
Let's examine the components of the definition of adaptation. The focus on reliably developing structure means that an adaptation tends to emer ge with regularity during the course of a person' s life. The mechanisms that allow humans to see, for example, develop reliably . But this does not mean that vision develops invariantly . The development of the eye can be perturbed by genetic anomalies or by environmental trauma. The emphasis on reliable development suggests that evolutionary approaches are not forms of "genetic determinism." Environments are always needed for the development of an adaptation, and environmental events can always interfere with or enhance such development.
The emphasis on meshing with recurrent structures of the world means that adaptations emerge from, and are structured by , the selective environment. Features of the environment must be recurrent over time for an adaptation to evolve. The venomous snakes must be recurrently dangerous, ripe fruit must be recurrently nutritious, and enclosed caves must be recurrently protective before adaptations to them can emerge.
Finally, an adaptation must facilitate the solution to an adaptive problem. An adaptive problem is anything that impedes survival or reproduction. Stated more precisely, all adaptations must contribute to fit ness during the period of time in which they evolve by helping an or ganism survive, reproduce, or facilitate the reproductive success of genetic relatives. In sum, adaptations emer ge from and interact with recurrent structures of the world in a manner that solves adaptive problems and, hence, aids in reproductive success.
The hallmark of adaptation is special design. That is, the features of an adaptation are recognized as components of specialized problem-solving machinery . Factors such as efficienc in solving a specific adaptive problem, precision in solving the adaptive problem, and reliability in solving the adaptive problem are key criteria in recognizing the special design of an adaptation. Adaptations are like keys that fit onl specific locks. The tines of the key (adaptation) show special design features, which mesh with the specific mirro -image elements within the lock (adaptive problem).
All adaptations are products of the history of selection. In this sense, we live with a stone-age brain in a modern world, which is in some ways dif ferent from the world in which we evolved. For example, ancestral humans evolved in relatively small groups of 50 to 150, using both hunting and gathering as methods of acquiring food (Dunbar , 1993). In the modern world, by contrast, many people live in lar ge cities surrounded by thousands or millions of people. Characteristics that were probably adaptive in ancestral environments—such as xenophobia, or fear of strangers—are not necessarily adaptive in modern environments. Some of the personality traits that make up human nature may be vestigial adaptations to an ancestral environment that no longer exists.
The evolutionary process also produces things that are not adaptations—such as byproducts of adaptations. Consider the design of a lightbulb. A lightbulb is designed to produce light—that is its function. But it also may produce heat, not because it is designed to produce heat but, rather , because heat is an incidental by-product, which occurs as a consequence of design for light. In the same way , human adaptations can also have evolutionary by-products, or incidental effects that are not properly considered to be adaptations. The human nose, for example, is clearly an adaptation designed for smelling. But the fact that we use our noses to hold up our eyeglasses is an incidental by-product. The nose was designed for smelling odors, not for holding up glasses. Notice that the hypothesis that something is a by-product (e.g., by holding up eyeglasses) requires specifying the adaptation (e.g., the nose) of which it is a by-product. Thus, both sorts of evolutionary hypotheses—adaptation and by-product hypotheses—require a description of the nature of the adaptation.
Noise, or Random Variations
The third product of the evolutionary process is evolutionary noise, or random variations that are neutral with respect to selection. In the design of a lightbulb, for
example, there are minor variations in the surface texture of the bulb, which do not affect the functioning of the design elements. Neutral variations introduced into the gene pool through mutation, for example, are perpetuated over generations if they do not hinder the functioning of adaptations.
An example of noise, or a random variation, is the shape of the human earlobe. Some people have long earlobes; others have short earlobes. Some lobes are thin; others are plump. These variations represent random noise—they do not af fect the basic functioning of the ear .
In sum, there are three products of the evolutionary process—adaptations, byproducts, and noise. Adaptations are the primary product of the selective process, so evolutionary psychology is primarily focused on identifying and describing human psychological adaptations. The hypothesis that something is a by-product requires specifying the adaptation of which it is a by-product. The analysis of by-products, therefore, leads us back to the need to describe adaptations. And noise is the residue of nonfunctional variation that is selectively neutral.
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