Serological Data and Phylogenetic Inference: The Problem of Rates of Change

Kirsch, J. A. W. (Department of Zoology, Univ. Kansas, Lawrence, 66044) 1969. Serological data and phylogenetic inference: the problem of rates of change. Syst. Zool., 18:296-311 [Immunology; cladistics, phylogeny, evolutionary rate; Marsupialial -Inferences about the evolutionary history of a group based upon studies of its living members can only be made utilizing assumptions about evolutionary rates. One such assumption current among biochemists and serologists is that biochemical evolution has been steady and the same in all lineages. If this were so, cladistic relationships could easily be inferred from phenetics. The assumption of strict time-dependency may be interpreted either as a deterministic or as a stochastic statement. In the former case, the form of phenetic groups is logically determined in ways incompatible with the results of serological studies; this is illustrated by examples from the comparative serology of marsupials. A simple computer model for the evolution of a group of hypothetical taxa of known phylogeny, operating according to the rules of a stochastic interpretation of time-dependency, generates data about those taxa very like empirical data on marsupials. However, the inferred cladistics of the hypothetical group are incorrect in many details, and this is indicated by the incompatibility of phenograms representing the affinities of the taxa with their known cladistics. It is not claimed that the model reproduces the conditions of antigen evolution, but that it does indicate that such evolution, if time-dependent, must be so in a stochastic rather than a deterministic sense; and further, that having introduced chance as a factor in the evolution of biochemical characters, one has only a probability of inferring the correct phylogeny of a group from the resulting data about the similarities of contemporaneous taxa. In recent years, considerable information about the affinities of living organisms has been gained through comparative study of the physicochemical and immunological properties of macromolecules, primary sequences of proteins and peptides, and base correspondences in DNA strands. Biochemists and systematists have understandably been anxious to interpret these results in evolutionary terms, but any such inferences about the history of a group from the affinities of its living members must be based on assumptions about the rates of change in the characters used to establish those affinities. If it is accepted that similarity reflects cladistic relationship, it follows that, in general, the more similar organisms have had the more recent common ancestors. And, if evolution were strictly time-dependent-that is, if the rates of change were steady and the same in all lineages-theni the cladistics of a group could be readily and exactly inferred from the phenetics of the living members. This view of the evolution of serological and biochemical characters is therefore most attractive, and has been espoused by a number of workers (e.g., Fitch and Margoliash, 1967; Sarich and Wilson, 1967; Dayhoff and Eck, 1968). On the other hand, Goodman (1962) has stressed the interplay of time and selection in determining change in the antigenic properties of proteins. In the course of an extensive comparative serological study of marsupials (Kirsch, 1968a), it became evident that a deterministic interpretation of time-dependent evolution could not possibly hold for this group of mammals, since the assumption logically limits the form of groupings in ways that were not met by the data. Recognition of this incompatibility between expectation and results led to consideration of an alternative interpretation of timedependency which more nearly accords with the data.