Genetic Variation and Differentiation in the Spotted Owl (Strix occidentalis)

Abstract

-We used starch-gel electrophoresis to investigate genetic variability at 23 loci in 107 individuals from seven populations of the Spotted Owl (Strix occidentalis). These populations sample all three currently recognized subspecies. No genetic variation was found in six populations from Oregon and California. Average heterozygosity in owls from New Mexico was 0.022. The low level of genetic variability will make it more difficult to monitor the genetics of this threatened species; the paucity of variation is possibly due to a small overall effective population size or bottlenecks in the past. At one locus there was a major allelic frequency difference between the Pacific Coast populations (S. o. caurina and S. o. occidentalis) and the allopatric taxon (S. o. lucida) found in New Mexico; our estimate of FST is 0.55. We believe the two allopatric populations have long been isolated, and it is probable that they represent two species. The data do not help elucidate the subspecific status of S. o. caurina. Received 11 December 1989, accepted 11 May 1990. OVER the last two decades, evolutionary and systematic biologists have devoted a major effort to assessing the extent of genetic variation within, and differentiation among, populations (Lewontin 1985, 1986). This has been true of many disciplines, including ornithology (for recent reviews, see Avise and Aquadro 1982, Avise 1983, Barrowclough 1983, Corbin 1983, Barrowclough et al. 1985, Barrowclough and Johnson 1988). These results have allowed biologists to make inferences about the evolutionary history of a species and, perhaps, to generalize about evolutionary processes (e.g. Lewontin 1974). In addition, these studies provide systematists with data, independent of classical morphology and phenotypes, to assess biogeographic and taxonomic patterns (e.g. Barrowclough 1985). Conservation biologists now realize that such data permit monitoring threatened populations for evidence of genetic deterioration (Lande and Barrowclough 1987) as well as identifying evolutionary and taxonomic units for conservation (e.g. Ryder 1986a, b). The recent report on genetic variation in the endangered New Zealand Kakapo (Strigops habroptilus; Triggs et al. 1989) is an example. One North American species that is the subject of much concern among conservationists, foresters, and wildlife biologists is the Spotted Owl, Strix occidentalis (e.g. Gutierrez and Carey 1985, Dawson et al. 1987, U.S. Dep. Agric. For. Serv. 1988). We report on an electrophoretic examination of genetic variation within and among populations of the Spotted Owl over much of its range. This study adds to the small, but growing, body of reports on the extent of genetic variation and differentiation in natural populations of birds; it also provides a survey of the population genetics of a species of particular environmental concern.