Larger bacterial populations evolve heavier fitness trade-offs and undergo greater ecological specialization

Abstract

30Evolutionary studies over the last several decades have invoked fitness trade-offs to explain 31 why species prefer some environments to others. However, the effects of population size on 32 trade-offs and ecological specialization remain largely unknown. To complicate matters, trade-33 offs themselves have been visualized in multiple ways in the literature. Thus, it is not clear how 34 population size can affect the various aspects of trade-offs. To address these issues, we 35 conducted experimental evolution with Escherichia coli populations of two different sizes in 36 two nutritionally limited environments and studied fitness trade-offs from three different 37 perspectives. We found that larger populations evolved greater fitness trade-offs, regardless of 38 how trade-offs are conceptualized. Moreover, although larger populations adapted more to their 39 selection conditions, they also became more maladapted to other environments, ultimately 40 paying heavier costs of adaptation. To enhance the generalizability of our results, we further 41 investigated the evolution of ecological specialization across six different environmental pairs 42 and found that larger populations specialized more frequently and evolved consistently steeper 43 reaction norms of fitness. This is the first study to demonstrate a relationship between 44 population size and fitness trade-offs and the results are important in understanding the 45 population genetics of ecological specialization and vulnerability to environmental changes.