Pervasive Fitness Trade-Offs Revealed by Rapid Adaptation to Shifting Population Densities in Large Experimental Populations of Drosophila melanogaster

Abstract

Trade-offs are an inherent feature of organismal biology and fundamental to the evolution of natural populations. Here, we use experimental evolution in large, genetically diverse populations of Drosophila melanogaster to directly measure the manifestation of trade-offs in response to fluctuating selection on ecological timescales. We first conducted a lab-based selection experiment to quantify a genome-wide signal of fluctuating selection elicited in response to shifting population densities and in the absence of fluctuating abiotic conditions. We then conducted an independent experiment to show that lab-based manipulations of population density can identify loci relevant to selection during population expansion and collapse in an outdoor setting, where multiple biotic and abiotic conditions fluctuate simultaneously. In concert, our data indicate a role of eco-evolutionary feedbacks and generic fitness trade-offs in the maintenance of variation in natural populations and show how a coarse-grained genetic architecture of adaptation can lead to predictable evolutionary change across settings. © 2026 John Wiley & Sons Ltd.