Fig. 1.

The observed fitness relations between strains TD10C and TD2 vary across the methyl-galactoside/lactulose resource axis and are also dependent on strain frequency. Strain TD2 is a derivative of the common laboratory strain of E. coli, and TD10C is isogenic to TD2 except for a region around the lactose operon. The operon was transduced in from a wild strain, is constitutive, and contains a permease which is 2.16 times more active that that in TD2 (Dykhuizen and Dean 1994). Fitnesses of TD10C were estimated using initial frequencies of 0–20% (rare; filled circles) or 80–100% (common; open circles) on various proportions of the sugars lactulose and methyl-galactoside. The total concentration of sugar entering the chemostat is held constant, and the percentage of methyl-galactoside refers to the percentage of the total sugar that is methyl-galactoside. Between 23% and 30.5% methyl-galactoside, the fitness of TD10C is > 1 when rare, and <1 when common. Here, TD10C and TD2 can coexist, maintained by frequency-dependent selection in a balanced polymorphism. The error bars designate 95% confidence intervals. When TD10C is common, fitness is not linear because the high frequency of TD10C alters the concentration of resource. This figure is redrawn from Fig. 9 of Lunzer et al. (2002).