Fig. 8. Complicated hypothetical relations between a phenotypic trait and the amount of running exhibited during the final week of a multi-week exposure to wheels. Mice from Control lines (gray squares) exhibit a positive and linear quantitative relation with the amount of wheel running, but this relation is lost in the Selected lines (black circles). Some traits, such as the amount of neurogenesis in the hippocampus [see fig. 2D (Rhodes et al., 2003)], actually show this sort of complicated pattern.