Genetic Divergence in Galvezia (Scrophulariaceae): Evolutionary and Biogeographic Relationships Among South American and Galápagos Species

Nineteen populations representing all species of Galvezia were examined for electrophoretic differentiation at 23 loci. Allozyme variation within G. leucantha is minimal, suggesting that a genetic bottleneck, possibly resulting from a single colonization event, accompanied establishment of Galvezia in the Galàpagos Islands less than 1.5 million years ago. South American species have higher levels of genetic polymorphism. Interspecific divergence patterns in Galvezia are characterized by many qualitative allelic differences, including one to six marker alleles for each species. Isozyme data support delimitation of four species. Geographic modes of speciation predominate. Allozyme data indicate that the Galàpagos endemic is not a recent derivative from a continental progenitor; rather, it is most likely the sister species to G. fruticosa or to a lineage of G. fruticosa and an undescribed species from southern Peru. It is hypothesized that the presence of six fixed, unique alleles in G. leucantha identifies a relictual genotype from an extinct South American progenitor. Biogeographic patterns and the presence of an allele shared between G. leucantha and two populations from the Sechura Desert indicate that northern Peru is the probable source region for the Galàpagos introduction. Estimates of elapsed divergence times and paleoecological data suggest that continental species diverged in the late Pliocene or during the Pleistocene as a result of geographic isolation caused by climatic and vegetational change.