References

Darwin, C. (1872/1965). The expression of the emotions in man and animals. London: Murray. Hecker, E. (1873). Die physiologie und psychologie des lachens und des komischen. Berlin: Dummler.

Harris, C., & Christenfeld, N. (1997). Humour, tickle, and the Darwin-Hecker hypothesis. Cognition and Emotion, 11, 103-110.

DARWIN'S EVOLUTION THEORY/EVOLUTION, THEORY/LAWS OF. = Darwinism = biological evolution, doctrine of. The English naturalist Charles Robert Darwin (1809-1882) and the Welsh naturalist Alfred Russel Wallace (1823-1913) independently formulated the basic features/aspects of the theory of evolution, which was first publicly presented in 1858 at a meeting of the Lin-naean Society (named in honor of the Swedish botanist and taxonomist Carolus Linnaeus, 1707-1778). In 1859, Darwin firmly established the theory of organic evolution known as Darwinism, and his name is better known than Wallace's today in connection with the origination of evolutionary theory. However, both men were exceptionally modest concerning "ownership" of the theory. At first, Wallace held that human evolution could be explained by his and Darwin's theory, but he later departed from Darwin on this point, asserting instead that a guiding "spiritual force" was necessary to account for the human soul. Wallace also considered "sexual selection" to be less important in evolution than did Darwin, holding that (unlike Darwin) it had no role in the evolution of human intellect. The theory of evolution holds that all naturally occurring populations are gradually and constantly changing as a result of natural selection that operates on individual organisms and varies according to their biological fitness. According to the theory, the process of evolution led to an enormous diversity in animal and plant forms where one of these lines evolved into hominids and, eventually, into humans. The implication of this biological theory for the discipline of psychology is that the human mind and behavior are as subject to natural law as is animal behavior (cf., pangenetic theory - Darwin's theory of heredity which holds that personal traits are transmitted from parents to the next generation via particles of each body organ, or part hidden in the spermatozoon and ovum of the parents; also, posits that mental traits, as well as physical characteristics, are inherited by pangenesis; thus,

Darwin viewed mental processes in humans and animals as products of evolution and a proper subject for scientific investigation). Darwin recognized that the evolutionary process is characterized by constant divergence and diversification where it could be likened to an enormously elaborate branching tree with living species represented by the tip of the branches, whereas the remainder of the tree denotes extinct species; it is estimated that as many as 98% of all species that ever existed are now extinct. One ramification of the branching tree analogy is that it is meaningless to place different species in an ordinal sequence from lower to higher. For instance, birds evolved from a line of reptiles different from those that evolved into mammals, and carnivores evolved along a different branch of the mammals than did primates. Therefore, birds, cats, monkeys, and humans do not form a continuum of evolution; they are distinct types of animals. Evolution has not been an orderly process that produced organisms of consistently increasing subtlety and complexity that culminated in the appearance of humans. Rather, the line of organisms leading to humans is only one branch among numerous other branches, and the human species, perhaps, does not deserve the universal evolutionary importance often given to it. Evolution is assumed, generally, to account for the variety of species on the earth today where (over millions of years) changes have taken place that are due to variation in the genes of a population and to survival and transmission of certain variations by natural selection. The law of natural selection is defined as the elimination of those individual organisms that are least well-adapted to the environment, with the survival and greater proportionate increase of those that are better adapted. The operative factor, according to evolutionary theory, is competition (or struggle) for existence where the result is survival of the fittest (cf., optimal foraging theory - refers to an organism's searching for food using strategies that are most efficient or cost-effective in terms of minimizing metabolic energy or maximizing Darwinian fitness). The phrase "survival of the fittest" was devised by the English philosopher/psychologist/sociologist Herbert Spencer (1820-1903) to describe the results of biological competition and is equivalent to the phrase "survival of the best adapted organisms." Darwin (1859) postulated that natural selection interacts with genetic variation so that the fittest members of the population contribute most significantly to the gene pool of subsequent generations. Rate of evolutionary change is determined by rate of advantageous mutations and intensity of selection pressures. The process of evolution produces new species (called speciation) when two or more populations of a species become separated and isolated from each other in different environments; such populations evolve differently and, thus, become different species. The process of adaptation occurs when the environment remains fairly constant, and the entire species becomes better suited to the environment through natural selection and, thus, behaviors as well as anatomical structures evolve through the mechanism of natural selection [cf., competitive exclusion principle, also called Gause's principle - named after the Soviet biologist Georgyi F. Gause (1910- ) - refers to the proposition that two distinct, but similar, species cannot occupy the same ecological niche indefinitely; the Red Queen hypothesis - named after the logic expressed by the character of the Red Queen in Lewis Carroll's 1872 book "Through the Looking Glass" - is the proposition that any evolutionary advance by one species is necessarily detrimental to other species in the same ecosystem, so that species are viewed as involved in a competitive evolutionary race whereby they must evolve continually just to survive and maintain their positions; convergent evolution - the development of similarities, not based on communality of descent, in two or more groups of organisms; a tendency of unrelated animals in a particular environment to acquire similar body structures that enable them to adapt optimally to the habitat; convergence theory - holds that individuals begin with hereditary givens or traits that are modified subsequently by environmental stimuli; and neural Darwinism theory - states that groups of neurons are selected by experience to form the foundation of cognitive operations - such as learning and memory - and where such selectionism is viewed as an explanation for the brain's functioning; also, it is the proposition that synaptic connections in the nervous system are shaped by competition where only those that are relatively useful are the ones that survive]. Evolutionary change does not need to be slow, gradual, and continuous, and there are not necessarily any "missing links" in the fossil record of the evolution of humans. Although evolution is a theory, it is a well-established one; it is not a hypothesis but a theory that is the end product of an empirical science that rests on masses of accumulated data. The terms evolution, evolutionary theory, and theory of evolution are used by most people as though they were synonyms and all indicating the Darwinian position. However, this pattern of usage tends to be misleading. Evolution is not theory but a fact; the gradualist position of origin of species by natural selection advanced by Darwin (Darwinism) is one attempt to explain that fact (cf., catastrophism/neo-catastrophism - the theory that gradual processes of evolution have been modified by the effects of great natural cataclysms). Defenders of creation-ism/creation theory (i.e., the doctrine that all things, including organisms, owe their existence to God's creation and not to evolution) often mistake disputes over the best characterization of the evolutionary process as indications that biologists themselves regard evolution as merely a "theoretical" concept (cf., transformation theory - states that one biological species becomes changed into another, basically different, species over the course of time). The influence of evolutionary doctrine in psychology has been both powerful and productive; it encouraged the study of individual differences, helped establish the fields of comparative psychology and behavior genetics, provided the useful concepts of adaptation, purpose, and function in 20th century psychology, and advanced the scientific study of developmental psychology. It is interesting to note that the theory of evolution is the only theory that is referenced and described in John Dewey's (1898) introductory psychology textbook. A comprehensive theory of evolution, called the modern synthesis or neo-Darwinism, was forged in the early 1940s and emphasizes the integration of the concepts of natural selection, gradualism, and population genetics as the fundamental units of evolu tionary change [cf., evolutionarily stable strategy - described by the English biologist John M. Smith (1920- ) and the American chemist/physicist George R. Price (19221975), refers to any hereditary pattern of behavior that is fixed where - when most individuals in a population adopt it - no alternative behavior pattern has greater "Darwinian fitness" and so none other is favored over it by natural selection; evolutionary bottleneck -refers to a sudden decrease in the size of a population, typically due to an environmental catastrophe, and results in a loss or decrease in genetic variability and adaptability - even if the population is able to recover its original size; and non-Darwinian evolution - refers to changes in the relative frequencies of genes in a population resulting from "neutral mutation" and not from natural selection; also called random/genetic drift]. The relatively new area of study called animal sociobiology, which is the application of principles from evolutionary and population biology to animals' social behavior, has invoked the modern synthetic theory of evolution. This approach has stimulated scientists from various disciplines to reexamine the evolution of social behavior and to reconsider how the principle of natural selection works in this context. See also DOLLO'S LAW; EMPATHY-ALTRUISM HYPOTHESES; HAWK-DOVE/CHICKEN GAME EFFECTS; LAMARCK'S THEORY; MENDEL'S LAWS/PRINCIPLES; NATURAL SELECTION, LAW OF; PARSIMONY, LAW/PRINCIPLE OF; WEISMANN'S THEORY. REFERENCES

Darwin, C. (1859). On the origin of species by means of natural selection. London: Murray.

Wallace, A. R. (1870). Contributions to the theory of natural selection. London: Macmillan.

Darwin, C. (1871). The descent of man and selection in relation to sex. London: Murray.

Spencer, H. (1892). The principles of psychology. New York: Appleton. Dewey, J. (1898). Psychology. New York: Harper & Bros.

Fisher, R. A. (1929/1958). The genetical theory of natural selection. New York: Dover.

Hodos, W., & Campbell, C. (1969). Scala naturae: Why there is no theory in comparative psychology. Psychological Review, 4, 337-350. Gruber, H. (1974). Darwin on man: A psychological study of scientific creativity. New York: Dutton. Wilson, E. (1975). Sociobiology: The new synthesis. Cambridge, MA: Harvard University Press. Denny, M. (1980). Comparative psychology: An evolutionary analysis of animal behavior. New York: Wiley. Stanley, S. (1981). The new evolutionary timetable. New York: Basic Books.

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