The plasma proteins of 29 species of albatrosses and petrels were electrophoretically separated in acrylamide gels to clarify relationships at the species-group to family-group levels. Little in the resulting data from 472 birds seriously contests the present classification of the Procellariiformes; much of the biochemical evidence supports, confirms, and clarifies the proposals of conventional taxonomic methodology. The biochemical data give fresh insights into the interrelationships of procellariiform taxa, and highlight intriguing new problems. Sex, season, age, and other sources of non-genetic protein variation are insignificant for taxonomic purposes. Proteins of value for comparison include the transferrins, some a and f3 globulins, albumins, prealbumins, and non-specific esterases. Genetic variations in the mobility of these proteins are useful at the genus-group level and below. Other proteins are monomorphic at genus and family level, and three are monomorphic in both number and mobility throughout the Procellariiformes; these are useful reference points for calibrating samples on different gels. One conspicuous a protein is absent in the Hydrobatidae but present in all other families; the implications of this are discussed. Polymorphic proteins at the population or species level were not detected; this conspicuous phylogenetic conservatism is discussed with regard to its possible evolutionary significance. Following a summary of the protein data, three categories of defined probability statements, based on the biochemical and other evidence, allow speculative comment on the evolutionary relationships and history of the taxa within the Procellariiformes. The value of further biochemical research into the marine birds is emphasised.
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