Both strong directional selection and faster development are thought to destabilize development, giving rise to greater fluctuating asymmetry (FA), although there is no strong empirical evidence supporting this assertion. We compared FA in sternopleural bristle number in four populations ofDrosophila melanogaster successfully selected for faster development from egg to adult, and in four control populations. The fraction of perfectly symmetric individuals was higher in the selected populations, whereas the FA levels did not differ significantly between selected and control populations, clearly indicating that directional selection for faster development has not led to increased FA in sternopleural bristle number in these populations. This may be because: (i) development time and FA are uncorrelated, (ii) faster development does result in FA, but selection has favoured developmentally stable individuals that can develop fast and still be symmetrical, or (iii) the increased fraction of symmetric individuals in the selected populations is an artifact of reduced body size. Although we cannot discriminate among these explanations, our results suggest that the relationship between development time, FA and fitness may be far more subtle than often thought.

In theDrosophila literature, selection for faster development and selection for adapting to high density are often confounded, leading, for example, to the expectation that selection for faster development should also lead to higher competitive ability. At the same time, results from experimental studies on evolution at high density do not agree with many of the predictions from classical density-dependent selection theory. We put together a number of theoretical and empirical results from the literature, and some new experimental results onDrosophila populations successfully subjected to selection for faster development, to argue for a broader interpretation of density-dependent selection. We show that incorporating notions of α-selection, and the division of competitive ability into effectiveness and tolerance components, into the concept of density-dependent selection yields a formulation that allows for a better understanding of the empirical results. We also use this broader formulation to predict that selection for faster development inDrosophila should, in fact, lead to the correlated evolution of decreased competitive ability, even though it does lead to the evolution of greater efficiency and higher population growth rates at high density when in monotypic culture.

Dry weight at eclosion, adult lifespan, lifetime fecundity, lipid and carbohydrate content at eclosion, and starvation and desiccation resistance at eclosion were assayed on a long-term laboratory population ofDrosophila melanogaster, and one recently wild-caught population each of four other species ofDrosophila, two from themelanogaster and two from theimmigrans species group. The relationships among trait means across the five species did not conform to expectations based on correlations among these traits inferred from selection studies onD. melanogaster. In particular, the expected positive relationships between fecundity and size/lipid content, lipid content and starvation resistance, carbohydrate (glycogen) content and desiccation resistance, and the expected negative relationship between lifespan and fecundity were not observed. Most traits were strongly positively correlated between sexes across species, except for fractional lipid content and starvation resistance per microgram lipid. For most traits, there was evidence for significant sexual dimorphism but the degree of dimorphism did not vary across species except in the case of adult lifespan, starvation resistance per microgram lipid, and desiccation resistance per microgram carbohydrate. Overall,D. nasuta nasuta andD. sulfurigaster neonasuta (immigrans group) were heavier at eclosion than themelanogaster group species, and tended to have somewhat higher absolute lipid content and starvation resistance. Yet, these twoimmigrans group species were shorter-lived and had lower average daily fecundity than themelanogaster group species. The smallest species,D. malerkotliana (melanogaster group), had relatively high daily fecundity, intermediate lifespan and high fractional lipid content, especially in females.D. ananassae (melanogaster group) had the highest absolute and fractional carbohydrate content, but its desiccation resistance per microgram carbohydrate was the lowest among the five species. In terms of overall performance, the laboratory population ofD. melanogaster was clearly superior, under laboratory conditions, to the other four species if adult lifespan, lifetime fecundity, average daily fecundity, and absolute starvation and desiccation resistance are considered. This finding is contrary to several recent reports of substantially higher adult lifespan and stress resistance in recently wild-caught flies, relative to flies maintained for a long time in discretegeneration laboratory cultures. Possible explanations for these apparent anomalies are discussed in the context of the differing selection pressures likely to be experienced byDrosophila populations in laboratory versus wild environments.