Population density plays an extremely important role in determining the evolution of certain attributes of populations. The earliest investigations in this direction were conducted by MacArthur and Wilson (1967), who analyzed the nature of selection pressures acting on populations at extreme densities. They designated the characteristic types of selection operating at low and high densities as r-selection and K-selection, respectively. The terms r-selection and K-selection may be taken to encompass, respectively, the selection pressures occurring in environments with density-independent and density-dependent population regulation (Mueller and Sweet, 1986). The theory dealing with the evolutionary consequences of extreme population density implies that K-selection should result, among other things, in an enhanced ability to compete successfully for limiting resources (MacArthur and Wilson, 1967; Southwood, 1976), and this phenomenon has been modeled by a number of workers (Charlesworth, 1971; Anderson, 1971; Anderson and Arnold, 1983; Asmussen, 1983).

The evolution of higher competitive ability in populations of Drosophila melanogaster subjected to K-selection has been demonstrated (Mueller, 1988). These experiments used competition for food among larvae as one of the indicators of competitive ability, and the results correlated well with the outcome of actual competition experiments in terms of differential survival of larvae from r-and K-selected lines when reared with larvae of a third strain (Mueller, 1988). Larval competition for food in Drosophila is of Lhe" scramble" type; the ablest competitor is one that can effectively consume food at the fastest rate. Thus, when food is limiting, different individuals will obtain different quantities in accordance with their competitive ability (Bakker, 1961). Effective consumption of food implies consumption of food in an amount sufficient to permit completion of development. Larval competitive ability thus depends on many factors, such as feeding rate, relative time spent on molting, minimum food requirement for pupation, initial weight, and resistance to crowding. Most of these factors themselves depend on a number of factors. Experimental studies indicate that, in laboratory populations of D. melanogaster, the outcome of larval competition for food depends primarily on the feeding rate (Bakker, 1961).