Evolutionary Genetics of Birds I. Relationships among North American Thrushes and Allies

--We have employed phenetic and cladistic approaches to analyze frequencies of electromorphs encoded by 25-27 loci in several species of North American thrushes (family Muscicapidae) and their relatives. In broad perspective both methods of data analysis yield similar summaries of probable relationships among these species. All four examined species of Catharus are nearly identical in electromorph composition. Hylocichla mustelina is phenetically and cladistically allied to members of Catharus. Both Turdus migratorius and Sialia sialis lie outside the Hylocichla-Catharus clade and are phenetically quite distinct from it and from each other. Despite its current placement in a distinct family, Mimidae, Dumetella carolinensis is cladistically and phenetically more closely allied to the Turdinae than is Regulus calendula, in some classifications a current member of Muscicapidae, subfamily Sylviinae. A major point of ambiguity in our data concerns the relative affinities of Turdus migratorius and Sialia sialis to the Hylocichla-Catharus clade. Phenetically, both are roughly equidistant to that clade, and cladistically our data can yield alternative interpretations. The levels of genetic similarity between muscicapid species at various stages of taxonomic divergence are comparable to previous estimates for other Passeriformes but are far higher than typical estimates for other vertebrate and invertebrate taxa of equal rank. Received 9 April 1979, accepted 20 August 1979. NOTWITHSTANDING the considerable work of Sibley and his colleagues on the egg-white proteins of more than 1,000 species of birds (Sibley 1970, Sibley and Ahlquist 1972), very little information exists on the genetic relationships of avian taxa. In a recent exhaustive review of the multi-locus protein electrophoretic literature, Nevo (1978, see also Powell 1975) listed studies conducted upon a total of 243 plant and animal species, yet only 10 species of birds had been examined by these techniques. This is surprising, as the utility of multilocus approaches in estimating genetic variability within and among avian species has recently been convincingly demonstrated by Smith and Zimmerman (1976), Barrowclough and Corbin (1978), and others. In this study, we examine protein products of 25-27 loci in all seven species of thrushes that regularly inhabit eastern North America. These species are placed in Muscicapidae, subfamily Turdinae, in the recent classification of Aves by Morony et al. (1975). Also reported here are data on two additional related species that provide "outgroup" comparisons for a cladistic analysis: Regulus calendula (Muscicapidae, Sylviinae) and Dumetella carolinensis (Mimidae). With respect to the examined members of Turdinae, determination of the generic relationship of the Wood Thrush (Hylocichla mustelina) to members of Catharus, Turdus, and Sialia has proven to be a particularly refractory problem (Dilger 1956, Bourns 1967, Hendrickson and Yow 1973, Gibson et al. 1976). We will examine this and other issues of systematic import. Perhaps of greater general significance is the fact that the data also yield estimates of mean levels of genetic differentiation between avian taxa at various stages of taxonomic and evolutionary divergence. These data can be compared to a large number of similar estimates previously obtained for other vertebrate and invertebrate classes (Avise 1974, 1976; Ayala 1975; Barrowclough and Corbin 1978). 135 The Auk 97: 135-147. January 1980 136 AvlsE, PATTON, AND AQUADRO [Auk, V01. 97