Fig. 3.

Schematic representation of trait evolution from a robust to non-robust state. (A) A stable petal number of four that is robust to a given perturbation is the ancestral character state in the Brassicaceae. Petal number is buffered within the normal range of developmental pathway activity, such that only very high or very low activity shifts the mean trait value outside the buffering zone and increases variance. (B) In comparison with most crucifers, C. hirsuta petal number is not buffered against the normal variation of developmental pathway activity in response to a given perturbation. As such, C. hirsuta shows a derived character state of petal number variation between zero and four. This could reflect an abbreviated buffering zone in C. hirsuta or a reduction in the normal range of C. hirsuta pathway activity such that it falls below the buffering zone found in most crucifers.