Macroevolutionary trends suggest repeated and predictable historical processes underlying biodiversity. Often such trends appear as suites of evolutionary phenomena such as asymmetrical rates of character evolution, coincident ages, and the magnitude of subsequent diversification following major adaptive transitions. Serpentine endemic floras offer a unique chance to examine the predictability of historical processes associated with the origin and maintenance of new taxa. In California, there are 246 serpentine endemic taxa (176 full species) in 93 genera, and 39 families, and many more tolerator taxa (those that occur on, but are not restricted to, serpentine soils). Here, we examine the timing and likelihood of alternative pathways to endemism in the Californian flora using a phylogenetic approach for over 20 genera. Specifically, we examine the number of times within and among lineages endemics and tolerators have arisen, the directionality of changes among character states (non-tolerator, tolerator, endemic), and the relative ages of closely related taxa of each type.

Results/Conclusions

We found that the transition direction to serpentine endemism is directional and mostly irreversible, from non-tolerator to tolerator to endemic. Also, the transition to serpentine endemism generally did not correlate with increased diversification rates. Finally, most serpentine lineages were found to be relative young (i.e., no paleoendemics older than 10 million years or less), with some notable exceptions. This examination of patterns of edaphic endemism provides insight into evolutionary history of specialization to serpentine soils in the California flora.