STATISTICAL TESTS FOR NATURAL SELECTION ON QUANTITATIVE CHARACTERS.

Abstract

Evolutionary biologists tend to ascribe phenotypic changes in populations to natural selection. Plausible hypotheses are regularly offered to explain how certain features of an organism are adaptive for its way of life. Failure to find an adaptive explanation is easily attributed to incompleteness of the study or pleiotropic effects of genes. More rarely, consideration is given to nonadaptive or maladaptive processes in evolution. Since mechanisms of phenotypic change other than natural selection are known (Van Valen, 1960), especially random genetic drift, the prevalent style of evolutionary logic begs the hypothesis of natural selection. It is therefore of interest to develop statistical tests on the pattern and rate of evolution of quantitative characters which can be used to distinguish natural selection from random genetic drift in natural and experimental populations. Pattern tests.-There are two basic types of pattern for which a null hypothesis of randomness may be tested. In natural populations, where there are no replications, the analysis sometimes suffers from lack of a control for relevant environmental variation in time. To test the null hypothesis of evolution by random genetic drift against the alternative hypothesis of natural selection, it is necessary to determine how much of the phenotypic changes are an individual developmental response to a changing environment and how much a result of genetic evolution. However, a time series from a natural population can be used to test the null hypothesis that the phenotypic changes form a random pattern. A variety of standard tests of randomness are available, based on the number and ordering of positive and negative changes, e.g. the sign test, the runs test. For application to fossil populations, it is notable