Serpentine soils are characterized by low levels of the essential plant nutrients nitrogen, phosphorus, potassium, and calcium, as well as high levels of iron, magnesium, and manganese, and toxic levels of chromium, cobalt, and nickel. Serpentine-derived soils have been found to be a significant impetus for plant speciation and endemism worldwide. The Santa Barbara Jewelflower, Caulanthus amplexicaulis var. barbarae (J. Howell) Munz (Brassicaceae), is a rare herbaceous annual plant restricted to an archipelago of serpentine exposures in the San Rafael mountains of central California, USA. Its sister taxon, C. amplexicaulis var. amplexicaulis S. Watson, has a more widespread distribution in southern California. C. amplexicaulis var. amplexicaulis is largely restricted to granitic soils. Intra- and interspecific phylogenetic relationships Caulanthus amplexicaulus var. barbarae and related taxa in the "Streptanthoid Complex" of genera (Streptanthus, Caulanthus, Guillenia) were examined using nuclear ribosomal internal transcribed spacer (ITS) and chloroplast trnL intron sequences. Phylogenetic hypotheses supported monophyly of subgenus Euclesia which includes the bulk of the serpentine endemics in the Streptanthoid Complex but showed that as a group, the serpentine tolerant taxa were nonmonophyletic, occurring in at least three distinct clades. Extant population structure and recent population dynamics in C. amplexicaulus var. barbarae were examined using a combination of microsatellite loci and nuclear ITS sequence variation. These studies suggest a role for fire in the long-term maintenance gene flow among isolated populations on serpentine outcrops. Although ecologically and geographically isolated, C. amplexicaulis var. barbarae and C. amplexicaulis var. amplexicaulis remain fully interfertile in artificial crosses. The existence of genetically compatible taxa with such distinct edaphic requirements presents a unique opportunity for intensive study of the genetic basis of tolerance to serpentine soils.

Key words: Caulanthus amplexicaulis, chloroplast trnL, edaphic, microsatellite, nuclear ITS, speciation