Rare and endemic species provide unique opportunities to study the principles and processes of speciation. The Streptanthus glandulosus complex (Cruciferae) is a group of serpentine endemics, most of which are rare or threatened. This complex represents a particularly striking example of the role of substrate in the divergence of populations and taxa. These annual plants occur on serpentine soils in disjunct populations in the Coast Ranges of California from Tehama County to San Luis Obispo County, isolated by intervening regions of unsuitable, non-serpentine habitat. this isolation, accompanied by inbreeding, may have resulted in the many different phenotypes identifiable among populations within each species. The phylogeny of the group is unknown and taxonomic delimitations are disputed: from three varieties of one species to 13 distinct species have been recognized. Thus, the S. glandulosus complex not only requires systematic study but it also offers an excellent forum in which to study speciation processes in serpentine endemics. Based on interpopulational crossing experiments, Kruckeberg (1957) showed a negative correlation between interfertility of populations of the complex and the distance by which they were separated. We propose to build on Kruckeberg's classic studies of isolation by distance using a combination of enzyme electrophoresis and restriction site analysis of chloroplast DNA to examine genetic divergence, systematic relationships, and evolutionary processes. The specific objectives are : (1) assess allozymic and cpDNA divergence among populations and correlate genetic divergence with geographic patterns; (2) construct a phylogeny of the complex using cpDNA restriction site data and use the phylogeny to evaluate putative examples of paleoendemism and neoendemism, recent derivation of restricted taxa from more widespread ones, and recurrent adaptation of serpentine tolerance; (3) estimate levels of interpopulational gene flow using allozymic data; and (4) compare levels of allozyme variability in restricted and widespread taxa. The integration of interfertility, allozymic, and restriction site analyses of the S. glandulosus complex offers an almost unparalleled venus from which to study speciation processes, and will provide the information necessary for sound management of the complex.
