Genetic variation and environmental variation: expectations and experiments

Abstract

Because the diversity of genetic systems (defined as those characteristics of the organism influencing the rate of genetic recombination) is much greater in plants than in animals, such systems have been of particular interest to students of plant evolution. Ever since the work of Darlington (1939), it has been realized that the evolutionary forces acting on genetic systems are likely to be different from those acting on more conventional morphological and physiological traits. For example, a particular chromosome number or recombination frequency may have very little direct impact on the physiological functioning of the organism, on its survival and fecundity, but may have a marked effect on the evolutionary potential of the descendants of that individual. As Stebbins (1950) states, ‘Hence in discussing the selective value of genetic systems we must consider primarily the advantages a particular system gives to the progeny of those who have it ... the immediate advantages or disadvantages of the system are of secondary importance.’ In the early works of Darlington, Huxley, Mather and Stebbins, the hypothesis was proposed that particular genetic systems result from a compromise between the need for constancy so as to preserve adaptation to the immediate contemporary environment and the need for flexibility in the face of changing environments to which the species will become exposed in the future. This idea was so forceful in its elegance and in its explanatory power that it became engrained in evolutionary biology more as a paradigm of how genetic systems actually do evolve, rather than as a hypothesis requiring rigorous formulation and testing.

We may think of the evolution of genetic systems as a course of evolution which, although running parallel to and closely integrated with the evolution of form and function, is nevertheless separate enough to be studied by itself.

(Stebbins, 1950)