Electrophoretic and isoelectric focusing analyses of liver proteins of the steamer-ducks, Tachyeres patachonicus, T. pteneres, T. brachypterus, and T. leucocephalus, show these species to be distinct genetically, with the latter three species being more closely related to one another than any one of them is to T. patachonicus. There is also significant differentiation among populations of T. patachonicus. Estimates of the average calculated heterozygosity per species are high: 0.185, 0.160,O.

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... 5, and 0.084, respectively, and observed heterozygosities are 0.115 I 0.090, 0.202 f 0.118, 0.201 i-0.085, and 0.080 f 0.069, respectively. The genetic distances of Cavalli-Sforza and Edwards, Nei, and Rogers were estimated and used with a Wagner tree algorithm to prepare a consensus tree based on 1,000 subsets of allelic frequency data prepared by bootstrapping over loci. Estimates of F" for comparisons among populations of T. patachonicus and estimates of Nm based on the distribution of private alleles indicate that gene flow between some pairs of populations is significantly reduced. Values of F" are much higher than those prev-iously found for other pairs of avian populations. Genotypic distributions that deviate significantly from equilibrium expectations are found for several of the polymorphic loci of one or more species.