Evolution and psychology—p. 1 Running Head: EVOLUTION AND PSYCHOLOGY [in press, American Journal of Psychology, May 4, 2006] Modern Application of Evolutionary Theory to Psychology: Key Concepts and Clarifications Aaron T. Goetz and Todd K. Shackelford Florida Atlantic University We are grateful to Gordon Gallup and two anonymous reviewers for comments and suggestions that improved this article. In addition, we thank David Buss, Dave Bjorklund, Josh Duntley, Steve Gangestad, Dev Singh, and Randy Thornhill for discussions of many of the issues addressed in this article. Address correspondence to Aaron T. Goetz, Florida Atlantic University, Department of Psychology, 2912 College Avenue, Davie, Florida 33314, United States; [email protected]. Evolution and psychology—p. 2 Abstract Darwinian selection has become the centerpiece of biology, and in the last few decades, many psychologists and anthropologists have recognized the value of employing an evolutionary perspective to guide their work. With a focus on evolved psychological mechanisms and associated information processing features, evolutionary psychology has risen as a compelling and fruitful approach to the study of human psychology and behavior. In this article, we review the instrument of evolution: natural selection, the products of evolution, and the impact of evolutionary thinking on modern psychological science. We conclude that as prejudicial barriers are overcome, as more evolutionary psychological work is conducted, and as hypothesized psychological mechanisms are substantiated in other disciplines, evolutionary psychology will emerge as the metatheory for psychology. Evolution and psychology—p. 3 Modern Application of Evolutionary Theory to Psychology: Key Concepts and Clarifications Darwin (1859) was not the first to suggest that species evolve. One of the first discussions of evolution predates Darwin by two and a half millennia. Anaximander, a Greek philosopher, suggested that, “in water the first animal arose covered with spiny skin, and with the lapse of time some crawled onto dry land and breaking off their skins in a short time they survived.” What Darwin (1859) provided, however, was a viable working mechanism for evolution: natural selection. Darwinian selection has become the centerpiece of biology, and in the last few decades, many psychologists and anthropologists have recognized the value of employing an evolutionary perspective to guide their work (e.g., Barkow, Cosmides, & Tooby, 1992; Chagnon & Irons, 1979; Daly & Wilson, 1983; Symons; 1979). With a focus on evolved psychological mechanisms and associated information processing features, evolutionary psychology has risen as a compelling and fruitful approach to psychological science. This article provides an introduction to evolution by natural selection and its modern application to the study of human psychology and behavior. Natural Selection and Sexual Selection Evolution by natural selection is the resultant process when a) individuals of a population vary in their characteristics, b) this variation is heritable, and c) individuals reproduce differentially (Darwin, 1859; Mayr, 1982). Individuals can vary morphologically, physiologically, psychologically, and behaviorally—no two individuals are exactly the same (even monozygotic twins vary). Due to these variations, some individuals may be better able to survive and reproduce in their current environment than other individuals. If the variations are heritable (i.e., if they have a genetic component), the characteristics can be passed from Evolution and psychology—p. 4 parents to offspring. Those individuals who have inherited characteristics that allow them to compete more effectively will produce more offspring. Thus, all organisms are subject to evolution by natural selection. As long as the ingredients of natural selection are present— variation, heredity, and competition resulting in differential reproduction—organisms will evolve. A classic example of natural selection follows. The peppered moth (Biston betularia) is typically white with black spots. This coloration provides an effective camouflage for the moths as they rest on certain Birch trees. There exists variation in the coloration of moths so that some are very white and some are very black. In a series of studies, Kettlewell (1955, 1956) documented that when the white trees on which the moths rested became dark from industrial pollution, birds ate more of the white moths because they were now conspicuous on the soot-covered trees. In polluted areas, the population of darker moths replaced the lighter moths, but in unpolluted areas, more of the light-colored moths had survived. Kettlewell showed that the environment in which the moths were better camouflaged contributed to better survival and reproduction. Kettlewell’s work is a classic demonstration of natural selection in action. Herbert Spencer’s summary of natural selection, “survival of the fittest,” has caused more confusion than clarification (Gaulin & McBurney, 2001). Reproduction is a much larger component of natural selection than is survival. If an individual had characteristics that enabled it to survive for hundreds of years, yet it never reproduced, those characteristics could not be favored by selection because without transmission to offspring, characteristics cannot become more common in a population. Survival, therefore, functions only to enable individuals to reproduce (directly or indirectly). Secondly, Spencer’s adage suggests that an individual may evolve to be the “fittest.” What determines an individual to be “fit” is its Evolution and psychology—p. 5 functional structure in relation to competing structures in the current environment. What is fit in one generation may be unfit in another generation. Also, fit is often taken to imply physically fit. Fitness, in an evolutionary context, is an organism’s success in producing offspring that survive to reproductive age (Williams, 1966). Sexual selection is the process that favors an increase in the frequency of alleles associated directly with reproduction (Darwin, 1871). Darwin distinguished sexual selection from natural selection, but today most evolutionary scientists include the two concepts under the label natural selection. Sexual selection includes the two components of intrasexual competition (competition between members of the same sex for sexual access to members of the other sex) and intersexual selection (differential mate choice of members of the other sex). Under sexual selection, even a trait that is a liability to survival can evolve. When the sexual attractiveness, for example, of a trait outweighs the survival costs needed to maintain it, the trait may be sexually selected. The epitome of a sexually selected trait is the peacock’s tail. Producing, maintaining, and maneuvering an unwieldy tail is metabolically costly for peacocks, and it is often the target of predators. The cumbersome tail evolved, however, because it was attractive to peahens. The mass and brightness of the plumage is attractive to peahens because it signals a modicum of parasites (Hamilton & Zuk, 1982). Peacocks with smaller, lackluster tails have been shown to be more susceptible to parasites and to have a higher parasite load. Thus, the large bright tail feathers are an honest signal of health, and peahens would be reproductively wise to select as mates males with such tails (who sire offspring that share their high quality genes). In many species, particularly polygynous species in which male reproductive variance is high and female reproductive variance is low, sexual selection is responsible for prominent Evolution and psychology—p. 6 sexual dimorphism. In such species, intrasexual competition between males for sexual access to females is fierce, and a size advantage is adaptive. It is often difficult to establish whether a trait evolved via natural selection or via sexual selection, but as mentioned previously, this distinction is not often necessary. In summary, the core premise of natural selection as a mechanism for evolution is that individual variation exists among traits in a population due to random mutations. Those individuals who have traits that better enable them to survive and reproduce will propagate the genes associated with those traits throughout the population. The Modern Synthesis and Inclusive Fitness Theory The details of modern evolutionary theory, or neo-Darwinian theory, are the result of the Modern Synthesis. From the early 1930s to the 1950s, advances in genetics, systematics, and paleontology aligned Darwin’s theory with the facts of genetics (Mayr & Provine, 1980). The Modern Synthesis is so-called because it was the integration or synthesizing of Darwinian selection with Mendelian genetics. R. A. Fisher, J. B. S. Haldane, Sewall Wright, Ernst Mayr, and Theodosius Dobzhansky are considered the primary authors of the Modern Synthesis (Mayr & Provine, 1980). With a more precise understanding of inheritance, Darwin’s theory of evolution by natural selection took flight as a powerful explanatory model. Following the Modern Synthesis, evolution by natural selection was extended once more to include inclusive fitness theory (Hamilton, 1964). Hamilton reasoned that selection could operate through classical fitness (i.e., the sum of an individual’s own reproductive success) and inclusive fitness, which includes the effects of an individual’s actions on the reproductive success of genetic relatives. That is, a trait will be naturally selected if it causes an individual’s genes to be replicated, regardless of whether the individual directly produces Evolution and psychology—p. 7 offspring. This addendum to natural selection produced a “gene’s eye” view of selection, and could now explain the evolution of altruistic behavior (i.e., behavior that is beneficial to others but costly for the actor). Genes associated with producing an alarm call when sighting a predator, for example, may spread throughout a population even when it is detrimental to the caller if the alarm call is emitted in the presence of genetic relatives and provides an overall benefit to those relatives (e.g., Sherman, 1977). Hamilton’s inclusive fitness theory is considered the most important advance in our understanding of natural selection, so much so that the term “inclusive fitness theory” is today synonymous with “evolution by natural selection.” The Products of Evolution: Adaptations, Byproducts, and Noise Although natural selection is not the only mechanism of evolution (e.g., mutation, migration, genetic drift), it is the primary means of modification and the only evolutionary force capable of producing functional organization (Fisher, 1954; Mayr, 1963; Williams, 1966). The creative force of natural selection, acting on random genetic variation, generates three products: adaptations, byproducts of adaptations, and noise. Adaptations are central to the study of evolution. Through the process of natural selection, small and incremental phenotypic changes that enhance an organism’s ability to survive and reproduce (relative to competing structures) accumulate to form an adaptation. Adaptations are inherited, they develop reliably, are usually species-typical, and were selected for because they were economic, efficient, and reliable solutions to adaptive problems (Buss, Haselton, Shackelford, Bleske, & Wakefield, 1998; Thornhill, 1997; Tooby & Cosmides, 1990; Williams, 1966). An adaptive problem is an obstacle or impediment that was recurrent during a species’ evolutionary history and whose solution affected the survival and Evolution and psychology—p. 8 reproduction (i.e., genetic propagation) of an organism. Furthermore, adaptive problems are not necessarily “problems;” instead, they are 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” (Tooby & Cosmides, 1992, p. 62). In sum, natural selection produces adaptations that solve problems associated with survival and reproduction. The heart, the production of sweat, and sexual arousal are all adaptations produced by natural selection. The heart is an anatomical adaptation that circulates blood throughout an organism’s body. The production of sweat is a physiological adaptation that thermoregulates an organism. Sexual arousal is a psychological adaptation that motivates sexual behavior. Not all products of natural selection are adaptations. Byproducts of adaptations are characteristics of a phenotype that are functionless and do not solve adaptive problems. They are called byproducts because they are incidentally tied to adaptations and are therefore “carried along” with them. Identifying byproducts is as equally as rigorous a process as identifying adaptations, because the hypothesis that a trait is a byproduct requires one to identify the adaptation of which it is a byproduct. The human navel and the whiteness of bone are byproducts of adaptations—they do not contribute in any known way to an individual’s survival or reproduction. In keeping with our mandate: The human navel is a byproduct of an umbilical cord and the whiteness of bone is a byproduct of the calcium in bones. The third product of evolution is noise, or random effects. Noise also is functionless and cannot solve adaptive problems. Noise can be produced by random changes or by perturbations in the genetic or developmental environment or by chance mutations. Noise, Evolution and psychology—p. 9 unlike a byproduct, is not linked to the adaptive aspect of a characteristic. The random shape of an individual’s navel is an example of noise. In summary, the evolutionary process generates three products: adaptations, byproducts, and noise. Adaptations are the product of natural selection and are functionally organized features that contribute to an individual’s reproductive success, however indirectly. Byproducts and noise do not solve adaptive problems and are not the direct products of natural selection. In the following section, we discuss how the study of psychological adaptations has changed the study of human psychology and behavior. Evolutionary Psychology Evolutionary psychology attempts to make sense of current human thought, emotion, and behavior by careful consideration of human evolutionary history. Over our evolutionary history, humans have faced many adaptive problems that needed to be solved to survive and reproduce. Generation after generation, over millions of years, natural selection slowly shaped the human brain, favoring circuitry that was good at solving these adaptive problems of our ancestors. The study of psychological adaptations (or evolved psychological mechanisms) is central to evolutionary psychology. Because the focus of evolutionary psychology is on describing adaptations, some have charged its practitioners with being “hyper-adaptationists.” Assuming a priori that a trait may be an adaptation, however, is a heuristic that guides research questions and methodology. Biologists have been conducting research this way for over 70 years. Moreover, byproducts and noise are typically only identifiable after the adaptations of which they are a byproduct or noise have been discovered and described (Tooby & Cosmides, 1990). Evolution and psychology—p. 10 Although modern evolutionary psychological theories are relatively new, all psychological theories are evolutionary in nature (Buss, 1995): “All psychological theories— be they cognitive, social, developmental, personality, or clinical—imply the existence of internal psychological mechanisms” (p. 2). The only known scientific explanation for complex functional organization—of psychological and physiological adaptation—in turn, is evolution by natural selection. Psychological Mechanisms as Information-Processing Modules. An evolved psychological mechanism is an information-processing module that was selected throughout a species’ evolutionary history because it reliably produced behavior that solved a particular adaptive problem (Tooby & Cosmides, 1992). Evolved psychological mechanisms are understood in terms of their specific input, decision rules, and output (Buss, 1995). Each psychological mechanism evolved to take in a narrow range of information—information specific to a particular adaptive problem. The information (or input) that the organism receives signals the adaptive problem that is being confronted. The input (either internal or external) is then transformed into output (i.e., behavior, physiological activity, or input relayed to another psychological mechanism) via a decision rule—an “if, then” procedure. An example of the input, decision rules, and output of a psychological mechanism is appropriate. Fruit can either be ripe or unripe. Because ripe fruit is more nutritious (i.e., calorically dense) than immature fruit, humans have evolved a preference for ripe fruit. The decision rule regarding the selection of fruit might proceed as follows: “If the fruit tastes sweet, then eat it.” If all fruit was maximally saturated with sugar all of the time, then that particular decision rule would not exist. The output associated with this mechanism might be to eat the ripe fruit or to discard the unripe fruit. This example illustrates the fact that psychological mechanisms can