AN EXPERIMENTAL STUDY OF GROUP SELECTION.

Darwin (1878) suggested in The Origin of Species (p. 230-233) that selection between families or communities had given rise to sterile castes in the social insects. He argued that sterility could not be favored directly by selection among individuals but that parents or communities might benefit, in terms of reproductive success, from the presence of sterile individuals. It is clear from his arguments that Darwin believed that selection among groups could result in a change in the types of individuals within a species. It was much later that Wright (1931) perceived intergroup selection as one of the primary forces controlling evolutionary change. He stated that the only practicable method of bringing about a rapid and non-self-terminating evolutionary advance would be through the subdivision of populations into isolated and differentiating groups, among which selection could be practiced. Haldane (1932) and Wright (1945) both considered the case of the evolution of altruistic behavior where individual selection acts against the altruist, but the altruist is favored by group selection. Group selection became a more controversial subject with the publication of Wynne-Edwards' book Animal Dispersion in Relation to Social Behavior (1962). It was Wynne-Edwards' thesis that group selection was a general phenomenon and was responsible for the evolution of mechanisms which regulated population size. Wynne-Edwards believed that population size was limited at some level below the "carrying capacity" (Errington, 1934) of the environment by means of individuals within the population withholding reproduction. To the extent that an individual withholds reproduction that individual suffers a loss of fitness, where fitness is measured by reproductive success. It was for this reason that WynneEdwards thought individual selection could not be responsible for the evolution of the mechanisms for withholding reproduction and therefore, some form of group selection was required. Territoriality, ritualized aggression, the establishment of hierarchies and other social orderings, the onset of senility, and the optimum reproductive effort were some of the regulatory mechanisms of populations that Wynne-Edwards proposed had evolved by means of group selection. The regulation of population size has been and continues to be an interesting and central problem in the field of population ecology and several authors, among them Andrewartha and Birch (1954), Williams (1964), Hamilton (1966), and Amadon (1964) have cogently discussed the evolution of the above mentioned regulatory mechanisms by means of individual selection. The focus of much of the controversy has been the question of whether or not group selection is required to explain the evolution of a particular character. In general, the viewpoint of most ecologists and theorists has been that individual selection is a faster and more efficient form of natural selection than is group selection for the following reasons: 1. In iteroparous species the generation time of the individual is short relative to the turnover time of the population and therefore selection proceeds faster when the unit of selection is the individual. 2. The numbers of individuals greatly exceed the numbers of populations. If the units of selection are numerous it is less likely that chance events will significantly influence the outcome of selection. There-