What is the prevalence of budding speciation in the CA Floristic Province?
Budding, or peripheral, speciation occurs when marginal populations become reproductively isolated from the remainder of the species. In organisms with high levels of local adaptation and population structure, such as plants, budding speciation is hypothesized to be particularly common. Regions with high habitat heterogeneity, like the California Floristic Province, are also thought to promote budding speciation, as populations of a species may interact with a variety of habitat types. Budding speciation results in “progenitor-derivative” species pairs, named such because one geographically widespread species retains the phenotypic and ecological niche of the ancestral species, whereas the other shows extensive divergence and occupies a smaller, abutting range.
Phylogenetic evidence is the most concrete way to identify progenitor-derivative relationships. We would expect the derivative species to be phylogenetically nested within the progenitor species, most closely related to the peripheral populations from which it evolved. However, lineage sorting and intraspecific gene flow within the progenitor will erase this signal over time, leading to two reciprocally monophyletic species. Using few loci to infer phylogenetic relationships can further reduce our ability to detect the signal of budding speciation, as not all gene trees may represent the true species tree.
We investigated a classically hypothesized case of budding speciation in the CA flora, specifically a serial budding speciation event among three Clarkia species native to the San Francisco Bay area and western coast. Clarkia franciscana is a federally-endangered and narrowly restricted serpentine endemic specie, occurring in only two serpentine regions in the Bay area. It’s hypothesized to be derived from C. rubicunda, a more widespread species in the Bay area that occurs on both serpentine and nonserpentine soils. Clarkia rubicunda, in turn, was hypothesized to be a derivative species of C. amoena, the most widespread species, occurring along the coast from the Bay area up to Washington state. Ecological divergence among the species, paired with asymmetric and abutting ranges, were primary factors in forming hypotheses of budding speciation.
We used a phylogenomic dataset to infer gene trees, species trees, and quantify gene tree discordance at nodes supporting progenitor-derivative relationships. The data suggested that this species trio did not evolve through serial budding speciation but instead underwent rapid divergence. Given variation among the species in chromosome rearrangements, one hypothesis is that variation in karyotypes among ancestral populations facilitated this rapid parapatric divergence. Moreover, this study motivates a critical eye with which we should view speciation histories more generally, given contemporary geographic patterns.
The Sianta Lab 