-The Puerto Rican gekkonids Sphaerodactylus nicholsi and S. townsendi have been alternately considered as separate species or subspecies of the taxon S. nicholsi. In distribution these taxa are largely allopatric, but the presence of apparent intergrade populations in southcentral Puerto Rico suggest that they are not reproductively isolated. Hybridization between these two taxa may have become possible when extensive agriculture created a patchy field-and-woods environment that resulted in a mosaic of proximate populations of S. townsendi, typically found in open coastal environments, and S. nicholsi, typically found in areas of dense vegetation and substantial ground cover. Electrophoretic data for 23 loci imply that highly restricted gene flow has occurred between S. nicholsi and S. townsendi without substantial genetic differentiation. The analysis of alleles in a suspected intergrade population suggests hybridization, and that S. nicholsi and S. townsendi are indeed not totally reproductively isolated. Several unique alleles were found in the suspected hybrid zone, and this phenomenon is discussed as it relates to hybridization. Although not conclusive, the data most strongly support the recognition of both S. nicholsi and S. townsendi as species. Among the taxa of the gekkonid lizard genus Sphaerodactylus on Puerto Rico are two relatively small forms, S. nicholsi and S. townsendi, which have been alternately considered as separate species or as conspecifics. Grant (1931) originally described them as separate species, S. nicholsi and S. townsendi. Thomas and Schwartz (1966) considered S. townsendi to be a subspecies of the nominate species S. nicholsi based on similarities of morphology, coloration and the presence of apparent intergrade populations. Later, Schwartz et al. (1978) ac5Corresponding address: R. W. Murphy, Department of Biology, UCLA, Los Angeles, California 90024, USA 6Present address: P.O. Box 101, Point Richmond, California 94807, USA. 7 Present address: 952 The Alameda, Berkeley, California 94707, USA. corded them both specific rank once again. Differences in body size (S. townsendi is usually larger than S. nicholsi), one versus two postnasals, and the presence or absence of a cephalic crescent appear to be the only characters separating these taxa. Although possible intergrade populations exist in southcentral Puerto Rico, the general distributions of these taxa are essentially allopatric, making inferences about potential interbreeding, and hence systematic conclusions, difficult. Initially we wanted to consider the questions of speciation and taxonomic status in the taxa S. nicholsi and S. townsendi using electrophoretic techniques. Electrophoretic data will not tell us unambiguously whether two populations are interbreeding or potentially capable of interbreeding, and as such these data do not address directly the pivotal This content downloaded from 157.55.39.35 on Wed, 31 Aug 2016 04:56:27 UTC All use subject to http://about.jstor.org/terms