No abstract

Very few experimental studies have examined how migration rate affects metapopulation dynamics and stability. We studied the dynamics of replicate laboratory metapopulations of Drosophila under different migration rates. Low migration stabilized metapopulation dynamics, while promoting unstable subpopulation dynamics, by inducing asynchrony among neighboring subpopulations. High migration synchronized subpopulation dynamics, thereby destabilizing the metapopulations. Contrary to some theoretical predictions, increased migration did not affect average population size. Simulations based on a simple non-species-specific population growth model captured most features of the data, which suggests that our results are generalizable. N atural populations often exhibit some degree of spatial structuring into metapopulations: ensembles of local populations (subpopulations) connected by migration (1). The effects of migration rate on the dynamics and stability of metapopulations have been extensively investigated theoretically (1). Analytical (2, 3) and simulation (4) studies have shown that even a simple system, consisting of two subpopulations (modeled by a pair of logistic maps) with a constant rate of to-and-fro migration, can exhibit rich dynamic behavior. In such systems, low, intermediate, and high migration rates have been shown to lead to complex, stable, and unstable dynamics, respectively (2-4). Similar results have been obtained with a variety of more realistic models (5-8). Potential stabilizing effects of migration have also been shown in studies on more complex systems (9-12). Although it has been empirically shown that migration can stabilize dynamics (13, 14), most metapopulation experiments have been carried out within the classical extinction-recolonization framework (15), which ignores the dynamics of population size. Thus, rigorous tests of theoretical predictions regarding the effects of migration rate on metapopulation dynamics are rare (13). Similarly, despite a large corpus of theoretical studies (16-18), the effects of migration rates on mean population size have rarely been investigated experimentally (19).Here, we report the effects of low (10%) and high (30%) migration rates on the dynamics of replicated laboratory metapopulations of the fruit fly Drosophila melanogaster in a 21-generation experiment (20). We quantified constancy stability (21) of the metapopulations and subpopulations with the use of a dimensionless measure of amplitude of fluctuation in population size over time (22). This statistic, which we call the fluctuation index (FI), is inversely related to stability. We also performed simulations (20) using a simple non-Drosophila-specific model to test whether the results reflect a simple effect of migration rates on typical population dynamics, or are due to some specific features of the life history and ecology of Drosophila cultures.Metapopulations with low levels of migration (henceforth LMMs) had lower FI values for metapopulation size than did either the control metapopulation...

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.

A series of laboratory selection experiments on Drosophila melanogaster over the past two decades has provided insights into the specifics of life-history tradeoffs in the species and greatly refined our understanding of how ecology and genetics interact in life-history evolution. Much of what has been learnt from these studies about the subtlety of the microevolutionary process also has significant implications for experimental design and inference in organismal biology beyond life-history evolution, as well as for studies of evolution in the wild. Here we review work on the ecology and evolution of life-histories in laboratory populations of D. melanogaster, emphasizing how environmental effects on life-history-related traits can influence evolutionary change. We discuss life-history tradeoffs - many unexpected - revealed by selection experiments, and also highlight recent work that underscores the importance to life-history evolution of cross-generation and cross-life-stage effects and interactions, sexual antagonism and sexual dimorphism, population dynamics, and the possible role of biological clocks in timing life-history events. Finally, we discuss some of the limitations of typical selection experiments, and how these limitations might be transcended in the future by a combination of more elaborate and realistic selection experiments, developmental evolutionary biology, and the emerging discipline of phenomics.

Four large (n > 1000) populations of Drosophila melanogaster, derived from control populations maintained on a 3 week discrete generation cycle, were subjected to selection for fast development and early reproduction. Egg to eclosion survivorship and development time and dry weight at eclosion were monitored every 10 generations. Over 70 generations of selection, development time in the selected populations decreased by approximately 36 h relative to controls, a 20% decline. The difference in male and female development time was also reduced in the selected populations. Flies from the selected populations were increasingly lighter at eclosion than controls, with the reduction in dry weight at eclosion over 70 generations of selection being approximately 45% in males and 39% in females. Larval growth rate (dry weight at eclosion/development time) was also reduced in the selected lines over 70 generations, relative to controls, by approximately 32% in males and 24% in females. However, part of this relative reduction was due to an increase in growth rate of the controls populations, presumably an expression of adaptation to conditions in our laboratory. After 50 generations of selection had elapsed, a considerable and increasing pre-adult viability cost to faster development became apparent, with viability in the selected populations being about 22% less than that of controls at generation 70 of selection.