The conclusions

``To account for divergence between inland and coastal California, we must invoke cumulative forces of selection that are 7 to 11 times stronger than the forces needed to account for differentiation of local populations.''

Furthermore, recall that the selection gradients can be used to partition the overall response to selection in a character into the portion due to the direct effects of that character alone and the portion due to the indirect effects of selection on a correlated character. In this case the overall response to selection in number of body vertebrae is given by

$${\bf G}_{11}\beta_1 + {\bf G}_{12}\beta_2 \quad ,$$

where ${\bf G}_{11}\beta_1$ is the direct effect of body vertebral number and ${\bf G}_{12}\beta_2$ is the indirect effect of tail vertebral number. Similarly, the overall response to selection in number of tail vertebrae is given by

$${\bf G}_{12}\beta_1 + {\bf G}_{22}\beta_2 \quad ,$$

where ${\bf G}_{22}\beta_2$ is the direct effect of tail vertebral number and ${\bf G}_{12}\beta_1$ is the indirect effect of body vertebral number. Using these equations it is straightforward to calculate that 91% of the total divergence in number of body vertebrae is a result of direct selection on this character. In contrast, only 51% of the total divergence in number of tail vertebrae is a result of direct selection on this character, i.e., 49% of the difference in number of tail vertebrae is attributable to indirect selection as a result of its correlation with number of body vertebrae.