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

The data represents fitness (measured as maximum growth rates) of  4 treatments with 6 replicate E.coli populations in each. The four treatments are:

GL: Evolved in Galactose at large population size.

GS:  Evolved in Galactose at small population size.

TL: Evolved in Thymidine at large population size.

TS:  Evolved in Thymidine at small population size.

These 24 populations were assayed in Galactose (G), Maltose (M), Sorbitol (S) and Thymidine (T) after scaling with the corresponding Ancestral growth values (given on a separate sheet in the same .xlsx file). All the figures in the paper are derivable from this data.