Feedback in Development and Its Evolutionary Significance

Abstract

A feedback system is one in which the deviation of a certain variable from a certain value causes controlling factors to oppose the deviation. The name denotes, not a class of explanatory principles, but a class of phenomena to be explained. Explanatory principles, mathematical in form, have been developed for certain general types of such systems. These are inapplicable to feedbacks involving the forms of structures, but can be applied to those which involve only quantitative variables. Three categories of quantitative feedback influence morphology: (1) Morphogenetic feedback, in which the size of a structure is the controlled variable; (2) Simple feedback in which the goal is a physiological state, controlled by variations in morphology; (3) Linked systems, in which a physiological state is maintained by changes in the rate of activity of a structure, while the rate per unit size is kept constant by changes in size. (2) and (3) ensure physiological stability in a variable environment, and tend to supersede morphogenetic systems in late development. The selective effect of a mutation changing the goal of a feedback will be the same in any environment in which the goal can be maintained, but that of a gene affecting a controlling factor will vary with the environment. Developmental feedback causes corrective pleiotropy, maintaining the physiological balance of organs in spite of mutations tending to disturb it. Genetic fixation may ensure that a developmental process is begun, but it cannot determine the end-point, which must adjust itself to the environmental conditions to which it is an adaptation. The environmental independence of higher organisms implies the independence only of certain physiological variables, and this is the result of the readiness with which precise and co-ordinated changes are imposed on many other variables, physiological and morphological, by the environment.

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