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Charles Robert Darwin is regarded as one of the greatest scientists who ever lived. He was the son of a wealthy provincial British doctor and entered into the study of medicine at Edinburgh University, but did not like it. He was then expected by his family to become a clergyman and graduated from Cambridge University. However, he was unenthused about pursuing a career in the church.
Fortunately for him, as well as the world of the biological sciences, he was offered an unpaid position assisting the captain of the British survey ship Beagle. From this little ship, Darwin spent the next five years exploring the natural world. His observations fostered a deep interest in biology and geology and motivated him to pursue the sciences as a career and eventually develop the theory of natural selection.
Charles Darwin’s theory of evolution by natural selection is the only scientific explanation for the nature and variety of life on our planet. Moreover, if life in other parts of our galaxy is governed by the same natural laws that govern it here, then Darwin’s logic also explains the verities of life in distant parts of our galaxy. However, it does not explain the origin of life itself. Darwin’s theory leaves the very beginning of life—the formation of the first primitive organic molecules—to chemists, physicists, and maybe even theologians. This entry focuses on the logic of how natural selection shapes the tools that enable organisms to survive the challenges of their environment, on some misconceptions about the theory of evolution, and on how evolutionary theory can help us understand human behavior. Finally, it briefly mentions some of the scientific and political controversies associated with current evolutionary thinking. Those interested in the vast evidence for Darwin’s theory should consult Mark Pagel’s Encyclopedia of Evolution (2002).
Darwin’s Finches and the Logic of Natural Selection
The Galapagos Archipelago is a group of islands off the coast of Peru. Eons ago, a single species of finch landed on one of the islands. Because resources for survival and reproduction were limited, the finches began competing for them. Some finches migrated to other islands. Since the environments of the islands differed, various biological tools, such as the shape of the birds’ beaks, differed in their effectiveness for dealing with these environments. Over time, fourteen species of finches evolved in the archipelago. Although the species vary in many ways, such as in size and coloration, differences in their beaks are particularly salient. The study of how the beaks of Galapagos finches contribute to survival and reproduction in the environments of the different islands has contributed greatly to the development of evolutionary theory (Weiner 1994).
Biological tools that contribute to an individual’s survival and reproduction are called adaptations. An adaptation is a trait—an anatomical structure, physiological process, or behavior pattern—that contributes more to an individual’s ability to survive, grow, or reproduce in comparison with alternate traits in the population. The large, powerful beak of the Geospiza magnirostris finch, useful for cracking large seeds, and the small, delicate beak of Certhidea olivacea, useful for extracting insects from the bark of trees, provide examples. Natural selection is the differential contribution of offspring to the next generation by genetically different members of a population, that is, the number of progeny of finches with genes for different beaks. The logic of natural selection can be explained in terms of assumptions and inferences that follow from the assumptions (Crawford 1998).
Assumption 1: The number of descendents of organisms in a population can grow exponentially.
Assumption 2: Resources enabling individuals in a population to exist can expand only arithmetically.
Assumption 3: The size of a population of individuals remains relatively stable across time.
Inference A: Competition for survival and reproduction ensues between individuals in a population.
Assumption 4: Individuals differ on traits that enable them to survive and reproduce.
Assumption 5: Some of the variation in these traits is genetic in origin.
Inference B: There is “differential contribution of offspring to the next generation by genetically different members of a population” (natural selection, by definition).
Inference C: Over many generations, “anatomical structures, physiological processes or behavior patterns that contribute more to individuals’ ability to survive, grow and reproduce in comparison with alternate traits in the population” (adaptations, by definition) are created.
In summary, some feature of the environment—for example, the arrival of a new predator or a change in climate—poses a problem for organisms in a population. A solution (e.g., longer legs, thicker fur) aids survival and reproduction. The above assumptions and inferences explain how natural selection provides the solution. Preexisting adaptations, sometimes called preadaptations, provide both stepping stones and limits to the solutions natural selection can provide (Pagel 2002).
Some Misconceptions about Darwin’s Theory
Assumptions 1, 2, and 3 do not require nature to be “red in tooth and claw,” as the poet Alfred Tennyson described it in In Memoriam (1850). A variety of subtle, and not so subtle, adaptations help individuals survive and reproduce. For example, animals in herds, such as wildebeests, may minimize the relative risk of predation by maneuvering to place other animals between themselves and predators (Hamilton 1971). Many types of camouflage coloration have evolved to protect organisms from their predators (Pagel 2002).
Although genes are involved in the development of all adaptations, assumptions 4 and 5 do not require that differences between individuals in evolved traits are genetically preprogrammed. Traits whose development is influenced by environmental factors are called facultative traits. Although male white-crowned sparrows, for example, cannot learn the song of another species, when they are nestlings they must hear an adult male of their own species sing and then themselves sing as juveniles if they are to sing a full song as adults (Konishi 1965).
Organisms do not evolve to act for the good of their species or the groups with which they live (Williams 1966). Since an organism’s time and energy are limited, an organism that helped a member of its species or group would pay a reproductive cost (i.e., have fewer offspring) to do the helping. Hence, one of its competitors who did not help would leave more offspring, and its kind would spread through the group or species at the expense of the helpers. However, modern Darwinists have two methods of explaining the widespread helping behavior seen in nature. The first is the evolution of cooperation, in which both parties increase their reproductive success by helping the other (Trivers 1971). The second is through helping genetic kin (Hamilton 1964). Genetic kin, such as brothers and sisters, have copies of genes that are identical to those of common ancestors. If a brother inherited a gene from his mother that predisposes him to help his sister produce offspring, that helping gene can spread through the sister’s offspring (his nieces and nephews) even though it reduces the brother’s own reproductive success. The reason is that the sister has a fifty-fifty chance of inheriting the same helping gene from their mother and passing it on to her children, her brother’s nieces and nephews.
Finally, natural selection does not have a goal. It is a purely mechanical process: traits that contribute more to survival and reproduction spread at the expense of traits that contribute less to survival and reproduction. Humans may evolve to be godlike creatures in the distant future. However, if this happens it will not be because natural selection had the goal of producing such beings.
Evolutionary psychology uses the principles of natural selection to understand the origin and functioning of the cognitive and emotional adaptations that helped us deal with problems in our ancestral environment, known as the environment of evolutionary adaptedness, and how those mechanisms function now (Crawford and Anderson 1989).
Mating of close genetic relatives, for example, can be detrimental to reproduction and survival because it brings together deleterious recessive alleles, such as those causing some genetic diseases, in the offspring of such mates. Some Darwinists have argued that intimate rearing of brothers and sisters during their first few years, which reduces or eliminates adult sexual attraction between them, may reflect one mechanism humans evolved to avoid this problem (Westermarck 1891). This argument is supported by evidence from: (1) boys and girls reared in the same children’s houses in Israeli kibbutzim, who rarely find each other sexually attractive as adults (Shepher 1983); (2) the reduced success of Chinese shim pau marriages, in which a genetically unrelated baby girl is adopted into a family at birth with the expectation that she will marry a son of the family at their sexual maturity (Wolf 1995); and (3) sexual attraction between adult genetic siblings who were separated at birth (Bevc and Silverman 2000).
In the upper panel of Figure 1, the assumption is that brothers and sisters with genes enabling them to develop psychological mechanisms to avoid incest had greater lifetime reproductive success across evolutionary time than those who did not. The result is the ancestral genotype for the avoidance mental mechanism. The ancestral developmental environment—intimate rearing with genetic siblings—produced the genetically organized ancestral cognitive and emotional mechanism(s) (the ancestral phenotype) that reduced sexual attraction between adult childhood intimates. The ancestral immediate environment refers to encounters between sexually mature, ancestral, opposite-sex individuals. Finally, natural selection favored the genes that enabled the development of the avoidance mental mechanisms.
The bottom panel represents an infinitesimal segment of evolutionary time—a few years in an Israeli kibbutz or a Chinese sim pau marriage or the meeting of an adult brother and sister who were reared apart from birth. In all cases, the putative adaptation functions as it evolved to function with respect to childhood intimates. However, because it functions in novel environments, its decision processes produce consequences that do not serve its evolutionary purpose. That is, it does nothing to prevent the mating of the genetic siblings, while reducing the likelihood of the success of the kibbutz and shim pau marriages.
Finally, evolutionary psychology is concerned with: (1) the problems that our hominid and primate ancestors encountered in their daily lives; (2) the psychological adaptations that natural selection shaped to help deal with those problems; and (3) the way the resulting evolved adaptations function in current environments (Crawford and Anderson, 1989).
Controversies about Darwinism and Human Behavior
Because of the great explanatory potential of Darwin’s theory of evolution, it engenders continuing scientific, religious, political, and social controversy. For example, if biologically based race, gender, and social class differences in anatomy, physiology, or behavior exist, then evolutionary theory helps explain their origin and significance (Degler 1991; Pagel 2002). However, the most salient issue—the one that underlies most controversies—is the degree of genetic specialization of evolved cognitive and emotional behavior–producing mechanisms. If natural selection produced primarily general-purpose psychological mechanisms, as many social anthropologists and social duced genetically highly-specialized psychological mechaactivists argue, then evolutionary theory is of limited use nisms, as most evolutionary psychologists argue (Barkow in understanding human behavior. However, if it pro- et al. 1992), then it is invaluable for understanding human mind and behavior and in developing solutions to social problems.
Table 1 shows the consequences for social policy of these two perspectives. The assumption underlying the table is that although specialized peripheral, informationprocessing mechanisms produce behavior, these specialized mechanisms can be assumed to develop from either a low or a high degree of innate genetic specialization of development. The columns under “Possible States of Nature” describe the possible states of the ancestral genotype shown in Figure 1. The rows indicate the two approaches to developing scientific explanations. Note that several different scientific approaches are listed in each row. The four cells in the table enumerate the outcomes of pairs of possibilities, that is, of pairing a high degree of genetic specialization with the scientific belief in a low degree of genetic specialization. Two cells describe valid outcomes. The two cells labeled “Evolutionary Psychologists’ Risk” and “Social Constructionists’ Risk” describe invalid outcomes. Many social activists, such as feminists and social constructionists, assume that the consequences of either making an evolutionary psychologist’s error or living in a society where ancestral evolved adaptations have an impact on current life, liberty, and happiness are so grave that they reject the possibility of the scientific explanations in the bottom row of the table.
Evolutionary psychologists reject this view. First, they worry about the suffering that could be caused by the social constructionist errors shown in Table 1. Second, they claim that accurate scientific knowledge about the human mind is crucial for developing more caring and harmonious societies. Third, they claim that natural selection has given the human species many evolved cognitive and emotional mechanisms, such as those underlying reciprocity and kinship, which if their functioning is understood can help us produce a better world. Fourth, they claim that evolutionary psychology is in fact an environmentalist discipline (Crawford and Anderson 1989)—the specialized psychological mechanisms that produce behavior, described in Figure 1, evolved to help humans deal with problems and stresses in their environments. Hence, understanding how these evolved cognitive and emotional mechanisms work can help us create better places to pursue life, liberty, and happiness.
- Barkow, Jerome, Leda Cosmides, and John Tooby. 1992. The Adapted Mind: Evolutionary Psychology and the Generation of Culture. New York: Oxford University Press.
- Bevc, Irene, and Irwin Silverman. 2000. Early Separation and Sibling Incest: A Test of the Revised Westermarck Theory. Evolution and Human Behavior 21: 151–161.
- Crawford, Charles. 1998. The Theory of Evolution in the Study of Human Behavior: An Introduction and Overview. In Handbook of Evolutionary Psychology: Ideas, Issues, and Applications, eds. Charles Crawford and Dennis Krebs, 3-42. Mahwah, NJ: Erlbaum.
- Crawford, Charles, and Catherine Salmon. 2004. The Essence of Evolutionary Psychology: An Introduction. In Evolutionary Psychology, Public Policy, and Personal Decisions, eds. Charles Crawford and Catherine Salmon, 23–50. Mahwah, NJ: Erlbaum.
- Crawford, Charles, and Judith Anderson. 1989. Sociobiology: An Environmentalist Discipline? American Psychologist 44: 1449–1459.
- Degler, Carl. 1991. In Search of Human Nature: The Decline and Revival of Darwinism in American Social Thought. New York: Oxford University Press.
- Hamilton, William D. 1964. The Genetical Evolution of Social Behavior, I and II. Journal of Theoretical Biology 7: 1–52.
- Hamilton, William D. 1971. Geometry of the Selfish Herd. Journal of Theoretical Biology 31: 295–311.
- Konishi, M. 1965. The Role of Auditory Feedback in the Control of Vocalization in the White-Crowned Sparrow. Zeitschrift für Tierpsychologie 22: 770–783.
- Pagel, Mark. 2002. Encyclopedia of Evolution. Oxford: Oxford University Press.
- Shepher, Joseph. 1983. Incest: A Biosocial View. New York: Academic Press.
- Trivers, Robert L. 1971. The Evolution of Reciprocal Altruism. The Quarterly Review of Biology 46: 35–57.
- Weiner, Jonathan. 1994. The Beak of the Finch: A Story of Evolution in our Time. New York: Knopf.
- Westermarck, Edward A. 1891. The History of Human Marriage. New York: Macmillan.
- Williams, George C. 1966. Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought. Princeton, NJ: Princeton University Press.
- Wolf, Arthur P. 1995. Sexual Attraction and Childhood Association: A Chinese Brief for Edward Westermarck. Stanford, CA: Stanford University Press.
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