Fitness evolution of T3 deleted for the ligase gene and adapted to the ligase-defective host (points A, B) or to the ligase-normal host (C). Final fitness equals initial fitness for the oblique line, thus the vertical distance from the oblique line to the labeled data point represents the total improvement during adaptation. Adaptation of A was initiated from T3Δ1.3A0, adaptations of B and C from T3Δ1.3B0. Initial fitness of T3Δ1.3B0 was negative on the ligase-defective host, so the phage could not initially maintain its numbers and had to be propagated initially on plates for the B adaptation. In contrast, initial fitness of T3Δ1.3A0 on the ligase normal host was over 20 doublings/hr. Initial fitness of T3Δ1.3A0 was higher, due to the acquisition of 5 changes by the first step at which the isolate could be identified as carrying the deletion. Final fitnesses of all three were substantially improved over initial. However, final fitnesses of A and B fell short of the fitness limit (28.9) of the ligase+ T3 adapted to the same host [designated limit (A, B)]. Adaptation of T3Δ1.3B0 to the ligase-normal host (point C) resulted in a final fitness approaching the presumed fitness limit. Standard errors in both the vertical and horizontal dimensions are indicated, often obscured by the symbols.