Fig. 3.

Mean ± SEM of the relative growth rate to day 14 for backcross hybrid and pure parental cross types on red maple (Left) and Bebb's willow (Right) foliage. Individual family means are presented (Upper) as well as the means of these family means (Lower). Each backcross type is circled and connected between panels by a dashed line to illustrate the ecologically dependent fitness of backcross hybrids (i.e., switching). This pattern documents extrinsic postmating isolation, a critical prediction of ecological speciation. The small number of families representing the pure parental willow cross type on maple reflects the general incapacity of this host form to survive on maple foliage. Different letters show significant differences from a Tukey HSD test at P < 0.05.