Journal of Heredity 1998;89:121–128 Low Genetic Variation Among Killer Whales (Orcinus orca) in the Eastern North Pacific and Genetic Differentiation Between Foraging Specialists

for genetic variation at three nuclear DNA markers and sequenced for a total of 520 bp from the mitochondrial control region. Two putative sympatric populations that range throughout this region were compared. They can be distinguished by social and foraging behavior and are known as ‘‘residents’ ’ and ‘‘transients.’ ’ We found low levels of variation within populations compared to other cetacean species. Comparisons between fish (resident) versus marine mammal (transient) foraging specialists indicated highly significant genetic differentiation at both nuclear and mitochondrial loci. This differentiation is at a level consistent with intraspecific variation. A comparison between two parapatric resident populations showed a small but fixed mtDNA haplotype difference. Together these data suggest low levels of genetic dispersal between foraging specialists and a pattern of genetic differ-entiation consistent with matrifocal population structure and small effective pop-ulation size. Over 1200 killer whales in the eastern North Pacific have been individually iden-tified using photographs of distinguishing congenital and acquired markings (Bigg e