NATURAL SELECTION AND RANDOM GENETIC DRIFT IN PHENOTYPIC EVOLUTION.

Abstract

In discussions of the major features of evolution, Simpson (1953) applied population genetic models to the interpretation of the fossil record. Most population genetics theory concentrates on details of the genetic system, such as gene frequencies and recombination rates, which cannot be directly observed or inferred from measurements on polygenic characters. Analysis of phenotypic data, particularly fossil material, requires models which are framed as much as possible in phenotypic terms. Starting from a simple formula of quantitative genetics, the methods of population genetics are used here to make a theory of the evolution of the average phenotype in a population by natural selection and random genetic drift. By analogy with Wright's (1931) adaptive topography for genotypes, Simpson (1953) proposed the concept of adaptive zones for phenotypes. This is an intuitive method of visualizing the dynamics of phenotypic evolution in terms of the degree of adaptation of the various phenotypes in a population, it usually being thought that natural selection increases adaptation. Such qualitative ideas are used by most evolutionary biologists and the notion of adaptive zones is popular among paleontologists. In the present paper, the concept of adaptive zones is clarified by the construction of an adaptive topography for the average phenotype in a population. This shows that with constant fitnesses the average phenotype evolves toward the nearest adaptive zone in the phenotype space. But if fitnesses are frequency-dependent the average phenotype may evolve away from an adaptive zone. A method is developed for estimating the minimum selective mortality necessary to produce an observed rate of evolution. In examples of the evolution of tooth characters in Tertiary mammals, these minimum selective mortalities are found to be exceedingly small. In his paper on the measurement of rates of evolution, Haldane (1949) stated that slowness of the rate of change makes it clear that agencies other than natural selection cannot be neglected because they are extremely slow by laboratory standards or even undetectable during a human lifetime. He briefly discussed mutation pressure. Random genetic drift due to finite population size is another such agency. The relative importance of natural selection and random genetic drift has been debated since Wright (1931, 1932) proposed that evolution is a stochastic process. Fisher (1958), for example, believed that random genetic drift is insignificant in relation to natural selection. The debate continues today at a more biochemical level (Lewontin, 1974). In order to objectively evaluate the role of random genetic drift in macro-evolutionary events, it is necessary to use mathematical models to determine the rate of evolution which can occur by repeated samplings of genetic material in a finite population. This paper presents a statistical test for the hypothesis of evolution by random genetic drift, contingent on the effective population size. In examples from the fossil record, it is found that rates of evolution equal to or greater than those observed have a significant probability of occurring by random genetic drift