Speciation in Sapsuckers (Sphyrapicus): II. Sympatry, Hybridization, and Mate Preference in S. ruber daggetti and S. nuchalis

Abstract

The Red-breasted Sapsucker (Sphyrapicus ruber daggetti) and Red-naped Sapsucker (S. nuchalis) are sympatric and hybridize in south-central Oregon, northeastern California, along the California-Nevada border, and in southern Nevada. We examined the overlapping distribution, nature of hybridization, and mate preference in these two taxa. Using a "hybrid index" system, we identified 13 phenotypic classes that represent the range of variation seen in typical parental types and their hybrids. Variation of parental forms in regions of allopatry was used to distinguish parental phenotypes from hybrids in the zone of overlap and hybridization. The percentage representation of various categories of mating (conspecific, 75.8%; backcross, 16.6%; hybrid, 1.4%; and interspecific, 6.2%) shown by 145 nesting pairs in the zone of overlap was used to infer the relative fitness of F1 and F2 generation hybrids. Although interspecific matings produce fully viable F1 offspring in numbers proportional to expectation, hybrid and backcross matings apparently are selected against. We suspect that F1 individuals and various recombinants have partial sterility barriers. In interspecific matings and in backcrosses, the male nearly always is S. r. daggetti or the redder mate. The data agree most closely with the dynamic-equilibrium model (stable-zone hypothesis), which has been proposed to explain zones of sympatry and hybridization in which gene flow from the extensive regions of allopatry of parental forms is balanced by selection against hybrids. In view of the preponderance of conspecific matings where S. r. daggetti and S. nuchalis occur together, they are regarded as biologic species. This decision is most appropriate for these taxa despite their low degree of hybridization and their near genetic identity as shown by electrophoresis.