A review of the Chitty Hypothesis of population regulation

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Abstract
Populations do not increase without limit, and 1 of the central problems of population biology is to explain why. The self-regulation hypothesis states that indefinite increase in population density is prevented by a change in the quality of the population. Changes in quality may be physiological or behavioral, genotypic or phenotypic, and 3 different mechanisms of self-regulation were proposed: the Stress Hypothesis suggests that mutual interactions lead to physiological changes, phenotypic in origin, that reduce births and increase deaths. The Behaviour Hypothesis suggests that mutual interactions involving spacing behavior prevent unlimited increase and that spacing behavior is not an inherited trait. The Chitty Hypothesis, or polymorphic behavior hypothesis, postulates that spacing behavior limits population density and that individual differences in spacing behavior have a genetic basis and respond to rapid natural selection. The testability of the Chitty Hypothesis is examined with regard to 13 predictions that are explicit in Chitty''s writings or derived by subsequent workers. Many of the predictions are not unique to the Chitty Hypothesis and only a few difficult manipulations adequately test Chitty''s proposed mechanism to the exclusion of all others. Four population studied are reviewed with reference to the Chitty Hypothesis. While the detailed mechanism proposed by Chitty is not yet adequately tested in any population, his general belief that both behavior and genetics are relevant to understanding population problems is now assumed.