The importance of contingency versus predictability in evolution has been a long-standing issue, particularly the interaction between genetic background, founder effects, and selection. Here we address experimentally the effects of genetic background and founder events on the repeatability of laboratory adaptation in Drosophila subobscura populations for several functional traits.We found disparate starting points for adaptation among laboratory populations derived from independently sampled wild populations for all traits. With respect to the subsequent evolutionary rate during laboratory adaptation, starvation resistance varied considerably among foundations such that the outcome of laboratory evolution is rather unpredictable for this particular trait, even in direction. In contrast, the laboratory evolution of traits closely related to fitness was less contingent on the circumstances of foundation. These findings suggest that the initial laboratory evolution of weakly selected characters may be unpredictable, even when the key adaptations under evolutionary domestication are predictable with respect to their trajectories.Adaptation, Drosophila subobscura, evolutionary contingency, founder effects, genetic background, life-history traits, repeatability.

The development of theory on density-dependent natural selection has seen a transition from very general, logistic growth-based models to theories that incorporate details of specific life histories. This transition has been justified by the need to make predictions that can then be tested experimentally with specific model systems like bacteria or Drosophila. The most general models predict that natural selection should increase density-dependent rates of population growth. When trade-offs exist, those genotypes favored in low-density environments will show reduced per capita growth rates under crowded conditions and vice versa for evolution in crowded environments. This central prediction has been verified twice in carefully controlled experiments with Drosophila. Empirical research in this field has also witnessed a major transition from field-based observations and conjecture to carefully controlled laboratory selection experiments. This change in approach has permitted crucial tests of theories of density-dependent natural selection and a deeper understanding of the mechanisms of adaptation to different levels of population crowding. Experimental research with Drosophila has identified several phenotypes important to adaptation, especially at high larval densities. This same research revealed that an important trade-off occurs between competitive ability and energetic efficiency.

Natural selection at high densities has often been postulated to favour the evolution of greater efficiency of food use. Contrary to this expectation, a previous study suggested the existence of a trade-off between larval feeding rate and efficiency at using food to complete larval development in populations of Drosophila melanogaster subjected to crowding for many generations. In this paper, we confirm the generality of such a density-dependent trade-off between food acquisition and utilization by demonstrating its occurrence in a new set of Drosophila populations subjected to extreme larval crowding. We suggest that such trade-offs between food acquisition and food use may represent a general phenomenon in organisms exhibiting scrambJLe competition. We test and reject the possible mechanistic explanation that decreased efficiency of food use in faster-feeding larvae may merely be a consequence of a faster passage of food through the gut, leading to incomplete assimilation of nutrients and energy.

Most demographic data indicate a roughly exponential increase in adult mortality with age, a phenomenon that has been explained in terms of a decline in the force of natural selection acting on age-specific mortality. Scattered demographic findings suggest the existence of a late-life mortality plateau in both humans and dipteran insects, seemingly at odds with both prior data and evolutionary theory. Extensions to the evolutionary theory of aging are developed which indicate that such late-life mortality plateaus are to be expected when enough late-life data are collected. This expanded theory predicts late-life mortality plateaus, with both antagonistic pleiotropy and mutation accumulation as driving population genetic mechanisms.

Density-dependent genetic evolution was tested in experimental populations of Drosophila melanogaster subject for eight generations to natural selection under high (K-selection) or low (r-selection) population density regimes. The test consisted of determining at high and at low densities the per capita rate of population growth of the selected populations. At high densities, the K-selected populations showed a higher per capita rate of population growth than did the r-selected populations, but the reverse was true at low densities.These results corroborate the predictions derived from formal models ofdensity-dependent selection. However, no evidence of a trade-off in per capita rate of growth was observed in 25 populations of D. melanogaster, each homozygous for a different second chromosome sampled from a natural population.Evolutionary ecology strives to understand-and, hence, to predict-the kinds of evolutionary change that different environmental conditions may bring about in populations. An important environmental variable is population density relative to essential resources. MacArthur and Wilson (1) examined this question by considering two alternative situations, called r-selection and K-selection. According to their predictions, natural populations commonly kept at low densities by density-independent mortality (and, hence, having abundant resources) should evolve high intrinsic rate ofgrowth (r), but be unable to have superior performance at high population densities. In contrast, populations usually living at high density (and, hence, experiencing strong competition for limiting resources) should evolve high intraspecific competitive ability and enhance their carrying capacity (K).Drawing from the theoretical work of ref. 2, Gadgil and Solbrig (3) have argued that r-selected species should devote a greater proportion of their resources to reproductive activities than K-selected species. According to ref. 4 the expected effects of r-and K-selection are indeed manifest over a broad range of taxa: r-selected species are characterized by small body size and a generation time shorter than one year, while K-selected species have larger body sizes and longer generations. A catalog of the phenotypes expected from r-and K-selection is given in ref. 5.The consequences ofdensity-dependent selection have been explored mathematically by several workers (6-10). Roughgarden (9, 11) assumes that fitness is equivalent to an individual's per capita contribution to population growth; fitness is further assumed to be a linear function of the total population size (N). population is often below its carrying capacity owing to frequent episodes of densityindependent mortality, the genotype with the highest r is favored. Thus, according to this model evolution favors the genotype that makes the highest per capita contribution to population growth, at either high or low densities depending on the environmental conditions. It has been pointed out by Stearns (5) that empirical work on the evolution of life history tra...