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Flowering time evolution is independent of serpentine tolerance in the California flora

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Comparative phylogenetic tests have been only recently applied to the many hypotheses about the role ultramafic (serpentine) soils play in the evolution and ecology of regional floras. An association between serpentine tolerance and early flowering has been observed at microevolutionary scales but not yet tested broadly across angiosperms. I used both hypothesis testing (phylogenetically independent contrasts) and model-selection approaches to compare published data of serpentine tolerance with flowering time using phylogenetic trees representing 24 clades of plants representing 27 genera and 17 families. A total of 126 independent contrasts revealed no significant difference in flowering times across all clades or within any one clade except for the Thelypodieae, in which flowering time of serpentine-tolerant lineages was later. Similarly, an Ornstein-Uhlenbeck model with one optimal flowering time was preferred over a model with separate optima depending on serpentine tolerance in nearly all genera. A phylogenetically un-corrected ANOVA found that serpentine-tolerant species have an earlier distribution of flowering times compared to both endemic and non-tolerant species, and median flowering times of non-tolerant species show a bimodal distribution. Therefore, I conclude that the long-term evolutionary responses of flowering time in a lineage that becomes tolerant of serpentine are variable across genera, with no significant overall bias toward earlier or later flowering. However, given that close relatives of serpentine-tolerant lineages tend to also flower relatively early, early flowering time may serve as an exaptation for serpentine tolerance. In combination with previously published ecotype studies, these results illustrate the eco-evolutionary scale dependence of flowering time.

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