Hybridization and Evolution of Oystercatchers on the Pacific Coast of Baja California

Abstract

The ranges of the Black Oystercatcher (Haematopus bachmani) and the American Oystercatcher {Haematopus palliatus frazari) overlap for approxi- mately 480 km along the Pacific coast of Baja California. Phenotypes are variable, especially in coloration, and hybridization has long been known. Yet, at most localities parental morphs predominate, hybrids are uncommon, and assortative mating occurs when possible. The historical record is sufficiently detailed to allow demonstration of changes in the hybrid zone. At the turn of the century- or whenever they were first studied- most populations were composed largely of parental morphs. After intensive col- lecting, resulting in the virtual extermination of some populations, recolonization occurred from both north and south. The reconstituted populations differed from the original ones, and by the 1920s and 1930s the frequency of hybridization was high. Recently, however, many populations have returned to their original composition. The prevalence of parental forms, a demonstration of assortative mating, and the resumption of stability in the zone of hybridization after a period of dynamic change, all show that there is selection against hybridization in this zone of secondary contact and that the two forms are specifically distinct. The nature of the selective forces remains to be determined.