The Phenogram as an Estimate of Phylogeny

Examination of a simple evolutionary model shows that the deficiencies of a phenogram, as an estimate of an historical cladogram, are not as great as sometimes imagined. In reality, there are grounds for a fair degree of confidence, particularly in relationships between taxa of higher rank. It can be shown, for instance, that uniformity in rates of evolution is not critical. What is required is, roughly, that later stocks should not evolve for substantial periods at rates much greater than in earlier stocks and/or that sister-species should diverge appreciably before undergoing further splitting. Likewise, the convergent elements of resemblance need not be small, or even uniform: simply, and roughly, later lines should not converge upon each other less than upon earlier lines. The latter holds also for "phylogenetic" reconstructions, based on what are believed to be resemblances in derived attributes only. There seem to be reasonable grounds for believing that such conditions are often met in Nature, and that a phenogram therefore supplies a reasonable phylogenetic hypothesis, for testing against further characters and taxa (but not methods) as they become available. This analysis does not conflict with the pragmatic aspects of classification, and can supply some criteria for choice of a numerical method; for instance, there are grounds for favouring "average linkage" as a clustering strategy. In a recent note (Colless, 1969), I pointed out that, under certain conditions, it becomes irrelevant whether matches in primitive attributes are included or excluded when reconstructing a cladogram (the branching pattern of a phylogeny) by phenetic methods. I was unable at that time to explore the situation in any detail; in fact, I drew several intuitive conclusions that were incorrect. Since then, Throckmorton (1968) has published an elegant formulation of a quantitative, "phylogenetic"l method; but he, like many other advocates of such methods, proceeds from the basic premise that, for credible reconstruction of a cladogram, primitive attributes must be identified, and as far as possible excluded from the outset. The situation therefore calls for further comment. I must here reiterate a basic thesis (Colless, 1967), which is essentially that of Bader (1958), that a "good" phenetic classification requires only the acceptance of evolution as a fact for it to become a "good" evolutionary classification. Mayr (1968, 1969) and, before him, Ghiselin (1966) 1 My usage of this term, and "phenetic," is d6scribed in Colless (1971). castigate the phenetic approach as based on a misinterpreted, nominalist or operationalist philosophy, which regards taxa as "constructed," rather than "discovered. " But that is a straw man-whatever the attitudes proclaimed by some of its proponents, the operations prescribed by phenetic method do not in any way require such a viewpoint. It is perfectly legitimate to describe them as the search for "natural" structure, and to interpret the results as fair estimates of phylogeny (see "Conclusion"). As in so many cases, what really matters is what we do, rather than the philosophy we proclaim while doing it. Throckmorton (1968) adopts the common and perfectly valid attitude that sharing of primitive attributes (Hennig's symplesiomorphy, which term I shall use here) does not constitute evidence of cladistic structure within the group for which they are "primitive. " Such matches contribute one element of "cladistic noise"; the other element is contributed by "convergence" (see below) and together they obscure the cladistic "signal. " "Noise" is of course being used here in a broad, non-technical sense, to mean interfering, irrelevant signals. A distinctive feature