418 Marc Ereshefsky
lizards snakes crocodiles birds
reptiles
Figure 2. Reptilia is paraphyletic because it contains lizards, snakes, and crocodiles
but not birds.
in the 20thCentury. But now many taxonomists object to their use in deter-
mining a taxon’s Linnaean rank [Eldredge and Cracraft, 1980]. They argue that
the concepts of ‘phenotypic diversity’ and ‘adaptive zone’ are imprecise and used
ambiguously. What counts as a distinct adaptive zone varies from phylum to phy-
lum. When it comes to phenotypic diversity, Hennig [1966, 156] playfully asks,
“whether the morphological divergence between an earthworm and a lion is more
or less than between a snail and a chimpanzee?” Most taxonomists now believe
that the concepts of ‘phenotypic diversity’ and ‘adaptive zone’ are too malleable
to serve as measures of a taxon’s rank.
Hennig [1965) offered his own definitions of the higher Linnaean ranks. He sug-
gested that taxa of the same rank originate during the same time period. Just as
geological strata are organized according to time of origin, biological taxa should
be assigned Linnaean ranks according to their time of origin. All taxa assigned the
rank of class, for example, are taxa that originated during the Late Cretaceous.
Orders would be defined as those taxa that originated during a more recent time.
Hennig’s suggested definitions of the Linnaean ranks are more precise than the def-
initions offered by evolutionary taxonomists. Unfortunately, Hennig’s definitions
are problematic from a cladistic perspective. (Recall that Hennig is the founder of
the taxonomic school cladism.) Taxa that originate during the same period often
have different phylogenetic or branching structures. Some taxa originating during
the Late Cretaceous are very successful and contain a number of subtaxa. Other
taxa originating during the same time are monotypic and contain only a single
basal taxon: they are phylogenetic twigs. From a cladistic perspective, Hennig’s
definitions place different types of taxa under a single rank. Consequently, cladists,
including later Hennig, abandoned the idea of correlating the ranks of taxa with
times of origin.
Neither evolutionary taxonomists nor cladists have established a universal cri-