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

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.