Enzyme Differentiation and Phylogeny in Clarkia Franciscana, C. Rubicunda and C. Amoena

When chromosomal reorganization in annual plants results in rapid speciation, the derivative species closely resemble their parents in morphological features and often in ecological tolerances.This suggests that divergence at the genetic level, i.e., the establishment of new alleles, is a much slower process than the erection of reproductive isolating barriers.The extent of genetic divergence at large numbers of loci can be established by gel electrophoresis of enzymes and other proteins.Several species of Clarkia furnish some of the most intensively studied examples of rapid speciation in annual plants (Lewis, 1962(Lewis, , 1966)).In order to investigate the genetic consequences of rapid speciation, electrophoretic studies of enzyme differentiation were initiated in Clarkia franciscana, C. rubicunda and C. amoena.Clarkia franciscana consists of a single population on a serpentine outcrop in San Francisco, California, and occurs near the northern periphery of the distribution of C. rubicunda.Clarkia amoena replaces C. rubicunda just north of San Francisco Bay and is found northward to British Columbia.The three species are closely similar morphologically but hybrids between any two of them have very low fertilities caused by meiotic irregularities attributed to gross chromosomal structural differences (Lewis and Raven, 1958a).Clarkia fran- siscana is morphologically uniform and highly self-pollinating, and has been observed (ibid.) to fluctuate in numbers from 600 to 4000 individuals in different years.In contrast, both of the other Clarkias generally outcross, consist of large numbers of populations, and are morphologically and ecologically variable.