Finally, phenotypic distinctiveness could be used to determine absolute ranks objectively (Mayr and Ashlock, 1991) if nature proceeded by discrete steps, or by gradual evolution under special circumstances (for instance, if cladogeneses were synchronous and if evolution proceeded at a steady rate). For instance, under a speciational or punctuated model of evolution, if the amount of phenotypic change could take only a few discrete values (not necessarily multiples of each other), phenotypic change could be used to assess the absolute rank of daughter lineages, at least for a small set of taxa (Fig. 2a). However, neither evolutionary theory nor observations corroborate any such evolutionary model; instead, the magnitude of phenotypic gaps appear to be highly variable (Fig. 2b). Even when the evolutionary model appears to be speciational or punctuated (Cubo, 2003; Mattila and Bokma, 2008), there is no evidence that the amount of phenotypic (or even genotypic) change takes a limited number of values, and of course, in most cases, gradual evolution presumably plays an important role, instead of, or in addition to, speciational change. Thus, phenotypic distinctiveness cannot be used to assess absolute ranks. It is difficult to show that phenotypic distinctiveness is highly variable between taxa, because it is difficult to quantify, but the very fact that it has not been quantified for most taxa (see Wills et al., 1994, for some exceptions) suggests that it has not been used as a criterion to rank taxa, or if used, only very imprecisely so.
If any regularity in the evolutionary model prevailed (if cladogeneses were synchronous, or if the tree were symmetrical, or if phenotypic gaps were discrete), ranks could perhaps be assigned objectively (Fig. 2c, d). However, as the above review shows, evolutionary theory and observations fail to confirm any of these special models. This leaves us without general rules to potentially assign ranks objectively.