Abstract
We have examined the effects of density and frequency in the larval competition of Drosophila melanogaster by measuring three fitness components: viability (V), mean development time (MDT) and a combination of these two (E). We have detected (contrary to most published results) non-linear effects of density in single-genotype cultures; in addition, different functions are required to describe the density effects below and above the optimal density. Frequency has also non-linear effects in the two-genotype cultures. Only one polymorphic equilibrium frequency, which is stable, occurs with respect to V; but two polymorphic equilibria, one stable and one unstable, exist with respect to E. The responses in single-genotype cultures do not allow one to predict the outcome of the competition in two-genotype cultures.
Highlights
The 'optimal density' (D) of a growing population with limited resources may be defined as that initial number of individuals that yields the maximum number of survivors (Wilson, 1980; Wallace, 1981)
In the case of larval competition studies, optimal density may be redefined taking into account viability (V) and mean developmental time (MDT), two fitness components showing density dependence (Barker & Podger, 1970; Caligari, 1980; Mather & Caligari, 1981)
Wallace's (1981) 'unit biological space' and Maynard Smith's (1974) territory are concepts that imply the existence of an optimal density for a group of organisms with a defined set of resources
Summary
The 'optimal density' (D) of a growing population with limited resources may be defined as that initial number of individuals that yields the maximum number of survivors (Wilson, 1980; Wallace, 1981). In the case of larval competition studies, optimal density may be redefined taking into account viability (V) and mean developmental time (MDT), two fitness components showing density dependence (Barker & Podger, 1970; Caligari, 1980; Mather & Caligari, 1981). The competitive fitness of different genotypes has been shown to depend on the density and the frequency of the competing genotypes (De Wit, 1960; Ayala, 1972; DeBenedictis, 1977; Harper, 1977; Mather & Caligari, 1981; Wallace, 1981; Tosic & Ayala, 1981). The experiments reported are designed to test this relationship with respect to optimal density, D, and to ascertain whether the density response is or not linear (Clarke & O'Donald, 1964; De Jong, 1976; Snyder & Ayala, 1979; Nunney, 1983)
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