A General Model to Account for Enzyme Variation in Natural Populations. II. Characterization of the Fitness Functions

A model of the genome is described which relates certain observations on the additive nature of enzyme activity to observations on the quantitative genetics of fitness traits. The model assumes that the enzyme activity of the heterozygote is intermediate between the activities of the associated homozygotes, and that a function, called the fitness function, maps enzyme activity onto Darwinian fitness. Evidence from the heterozygous effects of spontaneous lethal and mildly detrimental mutations supports the view that the fitness function is an increasing, concave function which approaches a finite limit. By assuming a particular representation of the fitness function it is possible to estimate its relevant parameter in two independent ways which show remarkable agreement. Two consequences of this model are explored. The first concerns the question of genetic polymorphism. If the enzyme activities are assumed to fluctuate at random in time and space, then conditions for polymorphism can be derived and they turn out to be fairly insensitive to the actual assumptions made about the structure of the environment. This suggests that progress can be made in understanding the forces maintaining polymorphism without detailed knowledge about the physical or biological environment. A second consequence of the model is that it can account for the observed vigor in F1 interpopulational hybrids. The vigor is shown to depend critically on the concave form of the fitness function.