In the first place it is necessary to be quite clear about what is meant by the term phyletic affinity. In the past, there has been much confusion because it has not been realized explicitly that the term is used for two quite distinct concepts. Firstly, there is relationship based upon that similarity between forms which can be attributed to a common ancestor. The more similar two forms are in their ancestral characters the more closely they are related. Alternatively, relationship is expressed in the genealogical sense of the number of generations which separate forms from their common ancestor: the more recently two forms had a common ancestor; the more closely they are related. The first type of relationship has been called "patristic affinity," the second "cladistic affinity". If evolution solely involved divergence of forms at a constant rate, then both types of evolutionary relationship would be the same; but it has long been recognized that rates of evolutionary change vary enormously, and two very different forms may well have had a recent common ancestor, whilst the common ancestor of two very similar forms may be remote, even in the absence of convergence. (Colbert, E. H.1949)
Much of the controversy that has raged over the evolutionary affinity of the australopithecines, for example, would seem to have arisen from failures to distinguish between patristic and cladistic affinity. Let it be assumed first that Australopithecus represents an early stage on the phylal lineage (or clade) leading to Homo and secondly that the hominid lineage has evolved more rapidly than the pongid lineage since their division. If the latter not unreasonable condition is satisfied, it is more than likely that the patristic affinity of Australopithecus is nearer the present day Great Apes than Homo, although "by assumption" its cladistic affinity is with the......
