Abstract
An electrophoretic comparison of proteins in 26 species of waterfowl (Anatidae), representing two major subfamilies and six subfamilial tribes, led to the following major conclusions: (1) the genetic data, analyzed phenetically and cladistically, generally support traditional concepts of evolutionary relationships, although some areas of disagreement are apparent; (2) species and genera within Anatidae exhibit smaller genetic distances at protein-coding loci than do most non-avian vertebrates of equivalent taxonomic rank; (3) the conservative pattern of protein differentiation in Anatidae parallels patterns previously reported in Passeriforme birds. If previous taxonomic assignments and ages of anatid fossils are reliable, it would appear that the conservative levels of protein divergence among living species may not be due to recent age of the family, but rather to a several-fold deceleration in rate of protein evolution relative to non-avian vertebrates. Since it now appears quite possible that homologous proteins can evolve at different rates in different phylads, molecular-based conclusions about absolute divergence times for species with a poor fossil record should remain appropriately reserved. However, the recognition and study of the phenomenon of apparent heterogeneity in rates of protein divergence across phylads may eventually enhance our understanding of molecular and organismal evolution.
Highlights
Species and genera within several families of passeriform birds exhibit far smaller genetic distances at protein-coding loci than do most nonavian vertebrates of equivalent taxonomic rank (Avise et al, 1980a, b, c; Barrowclough & Corbin, 1978; Corbin et al, 1974; Martin & Selander, 1975; Smith & Zimmerman, 1976)
In each of several passeriform families that have been extensively studied with multi-locus electrophoretic techniques, levels of genetic distance between congeneric species approximate those between conspecific populations of many fishes, mammals, and other non-avian vertebrates, and genetic distances between confamilial avian genera are typically lower than or equal to distances among very closely related species elsewhere (Avise & Aquadro, 1982)
Several hypotheses can be advanced to account for this conservative pattern of protein divergence in Aves: birds could be taxonomically 'oversplit' relative to other groups; hybridization and introgression between species could decelerate their differentiation; avian taxa could be younger than most other vertebrate taxa; avian proteins could be evolving more slowly, on the average, than those of other vertebrates
Summary
Species and genera within several families of passeriform (perching) birds exhibit far smaller genetic distances at protein-coding loci than do most nonavian vertebrates of equivalent taxonomic rank (Avise et al, 1980a, b, c; Barrowclough & Corbin, 1978; Corbin et al, 1974; Martin & Selander, 1975; Smith & Zimmerman, 1976). Several hypotheses (not mutually-exclusive) can be advanced to account for this conservative pattern of protein divergence in Aves: birds could be taxonomically 'oversplit' relative to other groups; hybridization and introgression between species could decelerate their differentiation; avian taxa could be younger than most other vertebrate taxa; avian proteins could be evolving more slowly, on the average, than those of other vertebrates These hypotheses are difficult to test critically, in part because the fossil record for most passeriform groups is very poor. We have examined electrophoretic variation in proteins encoded by 17-19 loci in a total of 206 specimens of 26 species of ducks, geese, and swans These samples represent several major lines of divergence in waterfowl, including two subfamilies and six distinct tribes. Even with these conservative estimates ot evolutionary age, protein differentiation in water. fowl appears to have proceeded at a very slow pace
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