Lancaster, L.T. and Kay, K.M. 2013. Origin and Diversification of the California Flora: Re-examining Classic Hypotheses with Molecular Phylogenies. Evolution 67: 1041-1053.
Do you love biodiversity? The diverse and beautiful California landscape? Angiosperm phylogenies? Then this paper is for you. The California Floristic Province (CFP) is a temperate biodiversity hotspot with over 5500 plant species, 40% which are endemic. How did this high plant species richness come to exist, and are elevated diversification rates in the state due to elevated speciation rates, decreased extinction rates, or both? The authors of this paper investigate these age old questions by performing three analyses on 16 angiosperm phylogenies of clades that are well-represented both in CA and beyond. Using an analysis of combined clades in BiSSE, they found that high diversification rates in CA versus elsewhere arose from a combination of low speciation rate and low extinction rate. When separating the clades into “Madro-Tertiary” and “Arcto-Tertiary” groups, which are the ancestral geofloras of California defined by Raven and Axelrod, they found that these floras migrated into California before other lineages, there was no difference in extinction rates between these groups. Lastly, when looking at the diversification rates of each of the 16 clades and the effect of the onset of the Mediterranean climate in CA, they found that only three clades experienced an increase in diversification rate when the climate changed, and that 5 of the 16 clades have actually experienced a decrease in speciation rates toward the present. It was surprising that these analyses found that a low extinction rate is likely the main driver of a high net diversification rate in CA, which implicates that the state’s topographic complexity and climatic buffering makes CA an important refuge under changing climates. However, one concern is that sampling was not even across the 16 phylogenies, as the proportion of species sampled for each clade ranged from 0.41 to 1.00 for CA species and 0.13 to 0.94 for non-CA species. Also, some of the phylogenies contained very few species from CA, making the data for non-CA species much stronger. Overall, the paper offers fresh, new insight into the underlying mechanisms driving high plant species diversity in the golden state. [UW phylogenetics seminar, 10/3/2013, Audrey Ragsac]