Turner, P. E., V. Souza, and R. E. Lenski. 1996. Tests of ecological mechanisms promoting the stable coexistence of two bacterial genotypes. Ecology 77:2119-2129.

Abstract:
A series of competition experiments with two genotypes of Escherichia coli showed that each genotype was favored when it was the minority, allowing their coexistence in a stable polymorphism. In these experiments, glucose was the sole source of carbon provided, and its concentration was limiting to population density. Thus, the stable polymorphism did not conform to a simple model of competitive exclusion. In similar experiments also with glucose as the sole resource, we considered two hypotheses that might explain the observed coexistence: (1) a strictly demographic trade-off, such that one genotype is competitively superior when glucose is abundant whereas the other genotype is the better competitor for sparse glucose; and (2) a cross-feeding interaction, whereby the superior competitor for glucose excretes a metabolite that acts as a second resource for which the other genotype is the better competitor. Although there was a demographic tradeoff, the advantage to the superior competitor at high glucose concentrations was too large off, the advantage to the superior competitor at high glucose concentrations was too large (given the initial concentration of glucose used in these experiments) to allow the second genotype to invade when rare at the observed rate. Therefore, the second genotype must have had some other advantage that allowed it to readily invade a population of the superior competitor for glucose. Indeed, the second genotype could increase in abundance after glucose was depleted, but only in the presence of the superior competitor for glucose thus implicating a cross-feeding interaction. These results confirmed earlier studies showing that populations of E. coli can maintain ecologically relevant genetic diversity even in a simple environment.