ON THE EVOLUTIONARY REVERSAL OF COMPETITIVE DOMINANCE.

The relationship between ecology and genetics is investigated, in part, by coevolutionary biology. One aspect of this relationship is the effect that genetic variability and natural selection have on determining the outcome of competitive interactions. This paper investigates the evolutionary reversal of competitive dominance, a particular mechanism whereby genetic considerations may influence competitive outcomes and population abundances. The formulation of these models was motivated by the laboratory experiments of Pimentel et al. (1965) with houseflies and blowflies and by the experiments of Ayala (1966a, 1969) and Moore (195 2a) with Drosophila. Arthur (1982) critically reviews these experiments. Pimentel et al. (1965) give a description of the dynamic interplay between the intensity of selection for competitive ability and the relative abundances of two competing species that forms the basis of the reversal of dominance. Suppose a rare and an abundant species are engaged in a competitive interaction. Members of the rare species will encounter the abundant species more often than they will encounter individuals of their own kind, hence superior interspecific competitive ability will be selected for. Conversely, individuals of the abundant species will encounter their own type more frequently, hence superior intraspecific competitive ability will be selected for. As the interspecific competitive ability of the originally rare species increases, the density of that species will increase and eventually overtake that of the originally abundant species, thereby producing a reversal of dominance. The most common experimental result of the authors cited above was for only one, or sometimes no, reversals of dominance to be observed, after which either the experiment was terminated, or one of the competitors went extinct. For example, in the experiment of Pimentel et al. (1965) houseflies and blowflies competed in a multicell laboratory cage. In nature, the houseflies and blowflies coexist on refuse and dung, while the laboratory populations oviposited on the same mixture of liver, yeast, agar and dried milk. During the first 50 weeks, or about 25 generations, houseflies comprised approximately 92% of the total fly population, and blowflies 8%. At about the twenty-fifth generation the blowfly abundance increased rapidly, with a concurrent decrease in the housefly population, which went extinct shortly thereafter. In single cell cages the blowfly usually went extinct, presumably before it could evolve increased interspecific competitive ability. Pimentel et al. therefore postulated that the spatial structure of the multicell cage allowed the blowfly to coexist sufficiently long for evolution, and hence a reversal of dominance, to occur. In this paper, I will use a quantitative genetical approach to modeling the evolution of species interactions and the reversal of dominance. The outline of the remainder of the paper is as follows: Three related mathematical models will be derived and discussed. In these models a distinction is made between the constant, nonevolutionary component of the competition coefficients, with a genetic variance of zero, and the part of the competition coefficients which is subject to evolutionary change, and therefore has a positive genetic variance. The first model is neutrally stable, and is used as a foundation on which the remaining two more