Experimental Evolution with Escherichia coli in Diverse Resource Environments: are 'Jacks of all Trades' truly 'Masters' of None?

Can a generalist population, evolved in two distinct resources, reach the same fitness in both as specialist populations, evolved in each resource individually, or, is a ‘jack of all trades’ truly ‘master’ of none? This question is relevant to theories of ecological specialization, the maintenance of genetic variation, and sympatric speciation, yet its answer remains uncertain, despite a wealth of experiments aimed at elucidating the limits of adaptation. To test whether bacterial jacks of all trades truly are masters of none, I measured the fitness, relative to a common ancestor, of replicate Escherichia coli populations evolved for 6,000 generations in the presence of either glucose or lactose alone (specialists), or in varying combinations (generalists). I found that all populations had significantly increased their fitness in both glucose and lactose, though the rate and magnitude of the increases differed. The generalists were masters of all trades for the first 4,000 generations; specifically, the geometric mean fitness of most generalist populations in both single-resource environments was not significantly different from the geometric mean fitness of the specialist populations measured in their selective environments. Subsequently, however, the generalists were masters of none as their geometric mean fitness fell increasingly behind the specialists at 5,000 and 6,000 generations. My results indicate that costs of adaptation are ultimately unavoidable, even if they fail to constrain the evolution of generalists for several thousand generations.