Fig. 2.

Chronogram showing estimated divergence times among volvocine algae. Colored boxes identify the 3 multicellular families; ingroup species not highlighted in this manner are unicellular (Paulschulzia pseudovolvox, the outgroup, represents a separate origin of multicellularity). Blue bars are the central 95% of estimates from 300 Bayesian posterior trees. Bayesian posterior probabilities <0.95 are shown. The green circle indicates the calibration estimated in the broad-scale analyses. Red letters indicate nodes referred to in the text. Character state changes are those supported by hypothesis tests in ref. 5. We have retained Kirk's (4) original numbering for these steps. *, steps 11 and 12 may have had 2 separate origins in the clade including V. africanus and V. carteri.