The Evolution of Serpentine Endemics: A Chloroplast DNA Phylogeny of the Streptanthus glandulosus Complex (Cruciferae)

Abstract

The Streptanthus glandulosus complex constitutes an ideal group for the study of evo- lution and edaphic endemism. The complex comprises both narrowly and widely distributed taxa, most of which are restricted to serpentine soils in the central Coast Ranges of California. Populations are typically highly disjunct, occurring on isolated outcrops of serpentine substrate. To investigate evolutionary patterns and infer processes of diversification in these edaphic endemics, we assessed chloroplast DNA (cpDNA) diversity among 55 populations within the complex and 11 additional species of Streptanthus and the closely related Caulanthus. Variation at 948 restriction sites yielded 12 most parsimonious trees. The strict consensus of cpDNA relationships suggests that the complex is monophyletic and consists of four major clades, each of which corresponds to a discrete geographic region. The levels and geographic patterns of cpDNA variation are consistent with the hypothesis that S. glandulosus subsp. glandulosus is a paleoendemic taxon that may be the progenitor of the entire complex. Some clades are not well resolved and include taxa that comprise only two or three populations; these taxa may have arisen relatively recently and could be considered neoendemics. These results complement and corroborate those from allozyme analyses and interpopulational crossing experiments. The evolutionary processes revealed in the S. glandulosus complex are likely to be common to many other endemics of the California Floristic Province, given the similar biogeographic histories of many species of the region. Despite the cosmopolitan occurrence of ser- pentine floras (Brooks 1987), surprisingly few studies have addressed hypotheses regarding the evolution of species endemic to serpentine soils. Stebbins (1942) classified endemics in general as either insular or depleted species, now considered neoendemics or paleoendem- ics, respectively (following Stebbins and Major 1965). A neoendemic is a taxon that has arisen relatively recently and has undergone minimal range expansion, whereas a paleoendemic is a taxon that has suffered range contraction, often leaving it with a disjunct distribution. We will show that the concepts of neoendemism and paleoendemism apply well to the evolution of serpentine plants. We will also demonstrate how the integration of phylogenetic and geographic patterns can be used to distinguish between neoendemics and paleoendemics and to reveal much about the routes to species rarity in both serpentine and non-serpentine environments. The unique physical and floristic character- istics of serpentine habitat have been apparent to naturalists for hundreds of years (Brooks 1987). Serpentine soils, which are derived from an array of ultramafic rock types, bear physical and chemical properties that render them un-