including comparative studies on different taxa, phenotypic manipulation, analysis of genetic correlations, and selection experiments, but most of these have interpretive limitations (Reznick, 1985; Futuyma and Moreno, 1988; Futuyma, 1998). Comparative studies, for instance, are essentially correlational, without access to knowledge about the ancestral condition or the evolutionary sequence of gains and losses of functions. Selection experiments, in which selection is imposed on one trait and correlated change is measured in another, are generally considered by evolutionary biologists to be the most powerful approach for demonstrating the existence of trade-offs (Futuyma, 1998; Sibly, 2002).

In this study, we use 24 experimentally evolved lineages of the bacterium Escherichia coli to analyze whether adaptation to low temperature (20°C) is accompanied by a loss of fitness at high temperature (40°C). We analyze these trade-offs from four perspectives:

1. Generality; is there a significant loss of fitness at high temperature across all lineages considered together?

2. Universality; do all of the lineages individually demonstrate a loss of fitness at high temperature?

3. Quantitative relationship; does the magnitude of adaptation influence the magnitude of trade-off? That is, do the cold-adapted lineages with the highest fitness at low temperature also have the lowest fitness at high temperature?

4. Historical contingency; does the prior thermal selective history of a lineage influence either the extent of its adaptation to low temperature or the magnitude of trade-off in fitness at high temperature?

Table 12.1 provides the measured fitness values at 20°C of each of the 20°C evolved lineages and its immediate historical progenitor, both obtained relative to their common ancestor. The resulting changes in fitness (ΔW) at 20°C are shown in Table 12.2. The temperature that each derived line had experienced before its evolution at 20°C had no significant effect on the extent of adaptation to 20°C (F_3,20 ± 0.497, P = 0.69). Given this absence of a historical effect and the absence of any other phylogenetic relationships among these lineages [star phylogeny from the common ancestral clone (Anc; Fig. 12.1)], we analyzed the significance of adaptation in all 24 lineages both individually and collectively. Mean ΔW for all 24 lines was 0.118 (±0.025 95% confidence limit, P < 0.0001), and ΔW was significantly positive (P ± 0.05 in Table 12.2) in 22 of the 24 lines.