FACTORS RESPONSIBLE FOR A CHANGE IN INTERSPECIFIC COMPETITIVE ABILITY IN DROSOPHILA.

Abstract

Classical interspecific competition theory assumes all individuals of a particular species are genetically alike in their ability to compete with another species. Recently, a number of studies have demonstrated the importance of the genotypes of competing species on the outcome of interspecific competition. A study by Lerner and Ho (1961) in Tribolium first showed that the genetic constitution of competing species was an important factor in determining competitive outcome. Other studies by Moore (1952), Pimentel et al. (1965) and Ayala (1966) have observed reversals of numerical dominance in competing species and suggested that genetic changes in competitive ability were responsible. If there is genetic variation for competitive ability, natural selection for increased competitive ability should occur when two competitors are artificially maintained in association. Such experiments for both intraand interspecific competition have had mixed results, although they appear to have been more successful when selection was for intraspecific competitive ability. Seaton and Antonovics (1967) in Drosophila and Bryant and Turner (1972) in Musca found significant increases in intraspecific competitive ability while van Delden (1970) observed increased interspecific competitive ability only after 65 generations in Drosophila. Selection for intraspecific competitive ability did not lead to a demonstrable change in experiments by Sokal et al. (1970) in Tribolium and Musca and for interspecific competitive ability in studies by Park and Lloyd (1955) in Triboliumn and by Futuyma (1970) and Barker (unpubl.) in Drosophila. Selection pressure for interspecific competitive ability for both competitors should be greatest when the two species are nearly equal in competitive ability and maintain similar numbers since at this point interspecific competition is maximized for both species relative to intraspecific competition. In an effort to find a strain of Drosophila melanogaster which could maintain nearly equal numbers to a v (vermilion eye) strain of D. siinulans, in 1968 J. S. F. Barker tested a number of mutant Drosophila melanogaster strains maintained by W. G. Baker. In a one generation test, he found that a yw (yellow body, white eye) strain of melanogaster produced a similar number of progeny to his simulans strain. Specifically, in mixed culture the melanogaster strain produced 5 9.0% of the progeny when 50% of the parents were melanogaster (Barker, 1971). The present study was initiated to examine interspecific competition between these two strains over an extended period of time using a serial transfer approach (Buzzati-Traverso, 1955; Ayala, 1966). Unexpectedly, melanogaster quickly eliminated simulans in interspecific competition. As a result the focus of this work became to (1) demonstrate whether or not there was a genetic change in the competitive ability of the melanogaster strain and then upon finding that there was, (2) determine in what respects the melanogaster strain changed such that its competitive ability was dramatically altered.