On the young age of intraspecific herbaceous taxa.

Dated phylogenies rarely include the divergence times of sister intraspecific taxa, and when they do little is said about this subject. We show that over 90% of the intraspecific plant taxa found in a literature search are estimated to be 5 million yr old or younger, with only 4% of taxa estimated to be over 10 million yr old or older. A Bayesian analysis of intraspecific taxon ages indicates that indeed these taxa are expected to be < 10 million yr old. This result for the young age of intraspecific taxa is consistent with the earlier observation that post-pollination reproductive barriers develop between 5 and 10 million yr after lineage splitting, thus leading to species formation. If lineages have not graduated to the species level of divergence by 10 million yr or so, they are likely to have gone extinct by that time as a result of narrow geographical distributions, narrow niche breadths, and relatively small numbers across populations.