Fig. 3. Example of a selection experiment (20 generations) with Drosophila melanogaster (Harshman et al., 1991) in which plasticity evolved to be higher in the Selected lines (S; N=3) as compared with the Control lines (C; N=3). C flies (left) were exposed to either standard medium or lemon for 24 h prior to sacrifice for measurement of a detoxification enzyme, and S flies (right) were similarly treated. Values are means ± s.d. For C lines, the magnitude of the induction caused by lemon exposure, as indicated by glutathionase S-transferase activity on lemon divided by the value on standard medium, was 1.16, whereas for S lines the value was 2.58. The greater induction caused by lemon exposure in the S lines relative to the C lines is an example of a genotype-by-environment interaction. TSO, trans-stilbene oxide. See text for further details.