DENSITY DEPENDENT SEXUAL SELECTION AND POSITIVE PHENOTYPIC ASSORTATIVE MATING IN NATURAL POPULATIONS OF THE SOLDIER BEETLE, CHAULIOGNATHUS PENNSYLVANICUS

In polygamous species, male reproductive effort is characterized by high variance in male reproductive success (Bateman, 1948; Trivers, 1972). The variance in male reproductive success can be partitioned into two components: variance in the number of females inseminated and variance in female reproductive success (Wade, 1979). The intensity of sexual selection on males is proportional to the variance among males in mating success, and this variance may be increased by female choice (Wade and Arnold, 1980). Consequently, selection favors characteristics in males which enhance attractiveness to females or promote dominance in competition for possession of females (Darwin, 1871). Fisher (1930) described the evolution of female preference and the sexually selected elaboration of the preferred male characteristic. Females that prefer to mate with males possessing some advantageous characteristic, not initially sexually selected, gain a selective advantage since their offspring will possess the character. Sons then carry alleles for the female preference as well as for the advantageous character. Therefore, preference is selected for through the selective advantage of sons. As the alleles for the preference spread, the preference lends an additional advantage to the possession of the preferred character. Fisher (1930) envisioned a runaway process in which the female preference and the preferred character evolve together. Thus, sexual selection may result in the evolution of sexual dimorphism. However, sexual selection on males may displace females from an adaptive peak, lowering mean population fitness (Lande, 1980). It is even possible for sexual selection to promote the extinction of a population (Lande, 1980). Theoretical models illustrate that a balance may occur between sexual selection and natural selection resulting in the maintenance of a stable polymorphism (O'Donald, 1967, 1977; Gadgil and Taylor, 1975; Karlin, 1978; Karlin and Farkash, 1978). Thus, factors which promote or retard the operation of sexual selection in natural populations may have an impact upon the genetic structure of these populations. Population density may influence the intensity of sexual selection. Eshel (1979), with a theoretical model, found high population density to favor the evolution of an attractive characteristic opposed by viability selection while at low population density viability selection was stronger regardless of the frequency of the attractive character. Alternatively, other models predict that female preferences will be less pronounced in high density populations (O'Donald, 1978). In an empirical study, Wade (1980) found that the variance in male reproductive success increased with density in laboratory populations of the flour beetle. The present study examines the relationship between population density and morphological characteristics of mating and nonmating individuals in natural populations of the soldier beetle, Chouliognathus pennsylvanicus Hentz. The morphological characteristics measured have been examined in previous studies of