success (classical fitness) plus the effects the individual’s actions have on the re-
productive success of his or her genetic relatives, weighted by the appropriate
degree of genetic relatedness.
It is critical to keep in mind that evolution by natural selection is not forward
looking or intentional. A giraffe does not notice juicy leaves stirring high in
a tree and ‘‘evolve’’ a longer neck. Rather, those giraffes that happen to have
slightly longer necks than other giraffes have a slight advantage in getting to
those leaves. Hence, they survive better and are more likely to live to pass on
genes for slightly longer necks to offspring. Natural selection acts only on those
variants that happen to exist. Evolution is not intentional and cannot look into
the future to foresee distant needs.
Products of the Evolutionary Process: Adaptations, By-Products, and Random Effects
In each generation, the process of selection acts like a sieve (Dawkins, 1996).
Variants that interfere with successful solutions to adaptive problems are fil-
tered out. Variants that contribute to the successful solution of an adaptive
problem pass through the selective sieve. Iterated over thousands of genera-
tions, this filtering process tends to produce and maintain characteristics that
interact with the physical, social, or internal environment in ways that pro-
mote the reproduction of individuals who possess the characteristics or the
reproduction of the individuals’ genetic relatives (Dawkins, 1982; Hamilton,
1964; Tooby & Cosmides, 1990a; Williams, 1966). These characteristics are called
adaptations.
There has been much debate about the precise meaning of adaptation, but
we offer a provisional working definition. Anadaptationmay be defined as an
inherited and reliably developing characteristic that came into existence as a
feature of a species through natural selection because it helped to directly or
indirectly facilitate reproduction during the period of its evolution (after Tooby
& Cosmides, 1992). Solving an adaptive problem—that is, the manner in which
a feature contributes to reproduction—is the function of the adaptation. There
must be genes for an adaptation because such genes are required for the pas-
sage of the adaptation from parents to offspring. Adaptations, therefore, are by
definition inherited, although environmental events may play a critical role in
their ontogenetic development.
Ontogenetic events play a profound role in several ways. First, interactions
with features of the environment during ontogeny (e.g., certain placental
nutrients, aspects of parental care) are critical for the reliable development and
emergence of most adaptations. Second, input during development may be re-
quired to activate existing mechanisms. There is some evidence, for example,
that experience in committed sexual relationships activates sex-linked jealousy
adaptations (Buss, Larsen, Westen, & Semmelroth, 1992). Third, developmental
events may channel individuals into one of several alternative adaptive paths
specified by evolved decision rules. Lack of an investing father during the first
several years of life, for example, may incline individuals toward a short-term
mating strategy, whereas the presence of an investing father may shift indi-
viduals toward a long-term mating strategy (e.g., Belsky, Steinberg, & Draper,
1991; for alternative theories, see Buss & Schmitt, 1993; Gangestad & Simpson,
642 D.M.Buss,M.G.Haselton,T.K.Shackelford,A.L.Bleske,andJ.C.Wakefield