Leroi, A. M., R. E. Lenski, and A. F. Bennett. 1994. Evolutionary adaptation to temperature. III. Adaptation of Escherichia coli to a temporally varying environment. Evolution 48:1222-1229.

Six lines of the bacterium Escherichia coli were propagated for 2,000 generations in a temporally varying environment. The imposed environmental regime consisted of alternating days at 32 degrees C and 42 degrees C, with rapid transitions between them. These derived lines are competitively superior to their ancestor in this variable temperature regime. We also measured changes in the fitness of these lines, relative to their common ancestor, in both the constant (32 degrees C and 42 degrees C) and transition (from 32 degrees C to 42 degrees C and from 42 degrees C to 32 degrees C) components of this temporally varying environment, to determine whether the bacteria had adapted to the particular constant temperatures or the transitions between them, or both. The experimentally evolved lines had significantly improved fitness in each of the constant environmental components (32 degrees C and 42 degrees C). However, the experimental lines had not improved in making the sudden temperature transitions that were a potentially important aspect of the temporally variable environment. In fact, fitness in making at least one of the transitions (between 32 degrees C and 42 degrees C) unexpectedly decreased. This reduced adaptation to the abrupt transitions between these temperatures is probably a pleiotropic effect of mutations that were responsible for the increased fitness at the component temperatures. Among the six experimental lines, significant heterogeneity occurred in their adaptation to the constant and transition components of the variable environment.