ALLOZYME VARIATION WITHIN AND BETWEEN LASTHENIA MINOR AND ITS DERIVATIVE SPECIES, L. MARITIMA (ASTERACEAE)

Enzyme electrophoresis was employed to examine genetic variation at 20 loci in 16 populations of Lasthenia minor and 18 populations of its presumed derivative species L. maritima. The purposes of the study were to ascertain levels of genetic variation in each species, to assess how the variation at enzyme‐coding genes is apportioned within and among populations of each species, and to determine the level of divergence between the two species. The two species are both diploid annuals, similar morphologically, and produce fertile F 1 hybrids when crossed. Lasthenia minor is self‐incompatible and restricted to mainland California, whereas L. maritima is self‐compatible and probably largely autogamous; it occurs on seabird rocks from central California to British Columbia. Mean genetic identities for pair‐wise comparisons of populations of the two species are similar to values for populations of the same species, indicating they have not diverged at the 20 genes coding for soluble enzymes. Despite its more extensive geographical range, L. maritima exhibits only 50% of the genetic diversity of L. minor. The latter species apportions a greater amount of its diversity within populations, whereas the former harbors more diversity among populations than within them. This is probably a reflection of the different breeding systems of the two species. Six unique alleles were detected in L. minor, whereas only one novel allele was found in a single individual of L. maritima. The electrophoretic data are concordant with the suggestion that L. maritima is relatively recently derived from L. minor. The switch from outcrossing to selfing and selection of genotypes adapted to the chemically and physically unusual substrate on the seabird rocks are considered the critical steps in the evolution of L. maritima.