Natural Insect Populations and Assortative Mating

Abstract

Numerous populations of two species of Coleoptera were examined from the standpoint of their tendencies toward phenotypic assortative mating with respect to various metrical characters reflecting size and coloration. Correlation coefficients for pairs found copulating in nature, population by population, reflected no consistent tendencies toward positive or negative assortative mating for any character, in either species. However, there were two color characters (one in each species) in which correlations deviated significantly from homogeneity, indicating, that preferences may not be uniform from population to population with respect to these characters. INTRODUCTION The subject of mating preferences among bisexual species, because of its relevance to questions such as the preservation of variability in natural populations and to speciation, has attracted the attention of evolutionists since almost the beginning of the science of population genetics. There has been no lack of work on theoretical aspects of assortative mating and its possible relationship to the maintenance of variation. (See, for example, Scudo and Karlin, 1969; Crow and Felsenstein, 1968; Crow and Kimura, 1970; Karlin, 1968; Workman, 1964, for recent treatments of a subject dating back to Wright, 1921.) Similarly, any number of valuable studies have been done on laboratory populations in which effects of artificially enforced assortative mating have been followed (including the classical work of Koopman, 1950, and Knight, Robertson and Waddington, 1956). As in most areas of population genetics, however, work on natural populations has not kept pace. There is so little information on mating preferences in nature that almost any generalization is dangerous. This is true even for insect populations, although here, relevant data can be gathered very conveniently. However, some interesting isolated cases are known from the insect world. A fascinating effect has been demonstrated in Panaxia dominula (Sheppard, 1952) where negative assortative mating has been invoked as the mechanism maintaining the medionigra gene in the Cothill population. Parsons' (1962) discovery of positive assortative mating in strains of Drosophila melanogaster derived from natural populations is of interest in what it may imply about the role that mating preference plays in speciation. For the most part, however, information available on insect populations (indeed, almost any populations) has shown, not whether assortative mating is of widespread occurrence in nature, but only that its presence would be most interesting, if found. The present study was designed to determine (for certain coleopterous species) 1. Whether phenotypic assortative mating can be detected in