The process of evolution by selection tends to use up heritable variation. In other words, heritable variants that are more successful tend to replace those that are less successful, resulting in species-typical adaptations that show little or no heritable variation. The universal human design is to have two eyes, for example.
In some contexts, two or more heritable variants can evolve within a population. The most obvious example is biological sex itself. Within sexually reproducing species, the two sexes exist in roughly equal numbers because of frequency-dependent selection. If one sex becomes rare relative to the other , evolution will produce an
increase in the numbers of the rarer sex. Frequency-dependent selection, in this example, causes the frequency of men and women to remain roughly equal.
Gangestad and Simpson (1990) ar gue that human individual dif ferences in women's mating strategies have been caused by frequency-dependent selection. They start with the observation that competition tends to be most intense among individuals who are pursuing the same mating strategy (Maynard Smith, 1982). This lays the groundwork for the evolution of alternative strategies.
According to Gangestad and Simpson, women' s mating strategies should center on two key qualities of potential mates—the parental investment a man could provide and the quality of his genes. A man who is able and willing to invest in a woman and her children can be an extraordinarily valuable reproductive asset. Similarly , independent of a man's ability to invest, women could benefit by selecting men wh have high-quality genes, which can be passed down to her children. Men may carry genes for good health, physical attractiveness, or sexiness, which are then passed on to the woman's sons or daughters.
There may be a trade-of f, however, between selecting a man for his parenting abilities and selecting a man for his genes. Men who are highly attractive to many women, for example, may be reluctant to commit to any one woman. Thus, a woman who is seeking a man for his genes may have to settle for a short-term sexual relationship without parental investment.
These various selection forces, according to Gangestad and Simpson (1990), gave rise to two alternative female mating strategies. A woman seeking a high-investing mate would adopt a restricted sexual strategy marked by delayed intercourse and prolonged courtship. This would enable her to assess the man' s level of commitment, detect the existence of prior commitments to other women or children, and simultaneously signal to the man her sexual fidelity and, hence, assure him of his paternit of future of fspring.
A woman seeking a man for the quality of his genes, on the other hand, has less reason to delay sexual intercourse. A man's level of commitment to her is irrelevant, so prolonged assessment of his prior commitments is not necessary . Indeed, if the man is pursuing a short-term sexual strategy , any delay on her part may deter him from seeking sexual intercourse with her , thus defeating the main adaptive reason for her mating strategy. This is referred to as an unrestricted mating strategy.
According to Gangestad and Simpson' s theory, the two mating strategies of women—restricted and unrestricted—evolved and are maintained by frequency-dependent selection. As the number of unrestricted females in the population increases, the number of "sexy sons" in the next generation also increases. As the number of sexy sons increases, however, the competition between them also increases. Then, because there are so many sexy sons competing for a limited pool of women, their average success declines.
Now consider what happens when the number of restricted females seeking investing men increases in the population. Because there are now so many women seeking investment, they end up competing with each other for men willing to invest. Therefore, as the number of women seeking investment increases, the average success of their strategy declines. In short, the key idea behind frequency-dependent selection is that the success of each of the two strategies depends on how common each strategy is in the population. As a given strategy becomes more common, it becomes less successful; when it becomes less common, it becomes more successful.
There is some evidence for this theory . Individual differences in female mating strategy (restricted versus unrestricted) have been shown to be heritable (Gangestad
& Simpson, 1990). Furthermore, there is some evidence to suggest the existence of two distinct female mating strategies. Finally, women who pursue an unrestricted sexual strategy have been shown to place more value on qualities of men linked with good genes, such as physical attractiveness and good health (Greiling & Buss, 2000). Additional research is needed on these important individual dif ferences in mating strategies, for they have important implications for social issues, such as father absence and single motherhood.
Another hypothesized example of personality dif ferences originating from frequency-dependent selection centers on psychopathy—a cluster of personality traits marked by irresponsible and unreliable behavior , egocentrism, impulsivity, an inability to form lasting relationships, superficial social charm, and a deficit in soci emotions such as love, shame, guilt, and empathy (Cleckley , 1988; Lalumiere, Harris, & Rice, 2001). Psychopaths pursue a deceptive "cheating" strategy in their social interactions. Psychopathy is more common among men than women, but psychopaths occur among both sexes (Mealey , 1995). Psychopaths pursue a social strategy of exploiting the cooperative proclivities of other people. After feigning cooperation, psychopaths typically defect, cheat, or violate the presumed relationship. This cheating strategy might be pursued by those who are unlikely to out-compete others in more mainstream or traditional social hierarchies (Mealey , 1995).
According to one evolutionary theory of this individual dif ference, a psychopathic strategy can be maintained by frequency-dependent selection. As the number of cheaters increases, and hence the average cost to the cooperative hosts increases, adaptations will evolve in cooperators to detect and punish cheating, thus lowering its overall ef fectiveness (Price, Cosmides, & Tooby, 2002). As psychopaths get detected and punished, the average success of the strategy declines. As long as the frequency of psychopaths is not too lar ge, however, it can be maintained amidst a population composed primarily of cooperators.
There is some empirical evidence consistent with this theory of the evolution of this individual dif ference cluster. First, behavioral genetic studies suggest that psychopathy is moderately heritable (W illerman, Loehlin, & Horn, 1992). Second, psychopaths often pursue an exploitative sexual strategy , which could be the primary route by which genes for psychopathy increase or are maintained (Rowe, 2001). Psychopathic men, for example, tend to be more sexually precocious, have sex with higher numbers of women, have more illegitimate children, and are more likely to get divorced if they marry than nonpsychopathic men (Rowe, 2001). This short-term exploitative sexual strategy would increase in populations marked by high geographic mobility, in which the costs to reputation associated with this strategy are muted (Buss, 2004). This leads to the alarming idea that we may be witnessing an increase in psychopaths in modern times, as society becomes increasingly geographically mobile. Recent evidence supports the frequency-dependent theory of this individual difference cluster—that it is part of normal personality variation, and is not due to "pathology" (Lalumiere et al., 2001). In sum, individual dif ferences in this cluster of personality traits—unreliability, egocentrism, impulsivity , superficial social charm and a deficit in empathy and other social emotions—may originate evolutionarily fro frequency-dependent selection (see also Millon, 1990, 1999, for additional explorations of personality from an evolutionary perspective).
The most recent ef fort to explore individual dif ferences from the perspective of frequency-dependent selection focuses on life history strategy (Figueredo et al., 2005a, 2005b). According to this approach, individuals have evolved dif ferences in the effort they allocate to reproductively relevant problems, such as survival, mating, and parenting. The core idea is that there are trade-of fs among these problems. Effort allocated to mating, for example, is ef fort taken away from parenting. On one end of the continuum, individuals favor what is called a K-strategy—greater ef fort is allocated to survival and heavy parenting over ef fort allocated to obtaining many mates. These high-K individuals are hypothesized to have formed strong attachments to their biological parents, avoid risk-taking that would imperil survival, pursue long-term mating rather than short-term mating, and invest heavily in children. Low-K individuals, at the other end, are hypothesized to have formed weaker attachments to their biological parents, have a risk-taking personality, pursue short-term mating, and invest little in their children. One study thus far supports the hypothesis that these variables do indeed covary or cluster together (Figueredo et al., 2005b). Future studies will be needed to determine whether individual dif ferences in K-strategy represent evolved frequency-dependent individual differences, but the approach appears promising.
In sum, we have examined several ways in which evolutionary psychologists study individual differences that might be adaptively patterned. First, dif ferent environments can direct individuals into dif ferent strategies, as in the case of father absence directing individuals toward a short-term sexual strategy . Second, there can be adaptive self-assessment of heritable traits, as is the case when individuals who are mesomorphic in body build pursue a more aggressive strategy than those who are ectomorphs. Third, two heritable strategies can be supported by frequency-dependent selection.
Fourth, the forces of selection can be dif ferent in different places, for example, or dif ferent times. This can result in evolved individual dif ferences that are due to different evolutionary selection pressures in dif ferent local ecologies. We know, for example, that individual differences in the presence or absence of "sickle cells" in the blood, an adaptation to protect against mosquito-borne malaria, have been caused by different selection pressures in dif ferent local ecologies. Although no individual differences in personality have yet been empirically traced to this particular evolutionary source, it remains a viable theoretical possibility in the evolutionary arsenal of explanatory options.
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