The effects of calibration age. A comparison of nrITS substitution rates with calibration age reveals higher variance at younger calibration points. The decreasing substitution rate with older calibration ages is only significant for the woody perennials. The herbaceous annual/perennial regression is not significant and becomes even less so when the outlier is removed (p = 0.861).
Kay et al. BMC Evolutionary Biology 2006 6:36 doi:10.1186/1471-2148-6-36