Genetic divergence among populations of a tropical passerine, the streaked saltator (Saltator albicollis)

--We analyzed mitochondrial-DNA (mtDNA) restriction-site variation in populations (subspecies) of the Streaked Saltator (Saltator albicollis) in Panama, Peru, and the Lesser Antilles. Genetic differentiation between populations (Panama vs. Peru, 0.035; Lesser Antilles vs. Panama/Peru, 0.063) greatly exceeded values reported for populations or subspecies within North American bird species (0.0028-0.0086), and was near the high end of the range reported for congeneric species of passerine birds (0.016-0.073). Nucleotide diversity within populations was similar to that reported for other species of passerines and did not differ markedly between mainland and island populations. Thus, founder effects and population bottlenecks associated with island colonization appear to have caused little, if any, loss of mtDNA nucleotide diversity. No significant mtDNA differentiation was observed between populations of named subspecies on mainland Panama (S. a. isthmicus) and the Pearl Islands (speratus), or between two subspecies in the Lesser Antilles (albicollis and guadelupensis). Saltator albicollis exhibits marked geographical genetic differentiation, as assayed by mtDNA polymorphism, but this bears little relationship to subspecies distinctions. Finally, the genetic data suggest that Antillean populations of Streaked Saltators should be distinguished at specific rank (S. albicollis), with continental forms referred to S. striatipectus. Received 7 May 1992, accepted 14 August 1992. MITOCHONDRIAL-DNA (mtDNA) sequence divergence based on restriction-site analysis (Avise et al. 1987, Dowling et al. 1990) has provided informative measures of genetic differentiation within avian species and genera (Mack et al. 1986, Avise and Zink 1988, Ball et al. 1988, Zink and Avise 1990, Quinn et al. 1991, Zink 1991, Zink and Dittmann 1991, Zink et al. 1991a, b, Bermingham et al. 1992). With regard to North American passerine birds, for example, mtDNA divergence between species is almost an order of magnitude greater than differences among populations within species. These findings refer exclusively to temperate taxa. At present, there is little information available on mtDNA divergence among populations of tropical birds, although allozyme surveys (Capparella 1988, Peterson et al. 1992) have revealed marked genetic differentiation over short distances within several groups of Amazonian and Mexican pas3 Mailing address: Smithsonian Tropical Research Institute, Unit 0948, APO, AA 34002-0948, USA. 4 Present address: Illinois Natural History Survey, 607 East Peabody Drive, Champaign, Illinois 61820, USA. serines. Additionally, Escalante-Pliego (1991) has documented substantial isozyme differences among disjunct populations of yellowthroats (Geothlypis) in Central and South America, and Hackett and Rosenberg (1990) have demonstrated pronounced geographical subdivision and differentiation in Neotropical antwrens (Formicariidae). Appraisals of genetic differentiation among populations of tropical birds would address questions of population structure and incipient species formation in regions of high species richness. Tropical species diversity is thought to result from high rates of species production, low rates of extinction, or great age of tropical regions. Species production presumably varies in relation to rate of evolutionary change within populations and degree of population subdivision. Estimates of mtDNA divergence directly address the issues of population subdivision and the relative genetic differentiation of taxa of specific rank. It has been suggested that the constant temperatures and diminished seasonal change in climate in the tropics have led to increased ecological specialization, increased fragmentation of species