3 The New York Years (1931–1953)

Classification 325
of anagenetic divergence which leads to ecological changes. It is a misleading
formulation to say that higher taxa split.
(6) A misleading conceptualization of ranking: Rank is given by cladistic procedures
automatically by time of origin, and the same rank must be given to
sister groups. This proposition is totally unacceptable and was later given up
or modified by cladists. The extremely different evolutionary fates of phyletic
lineages derived from the same stem species would lead to highly unbalanced
classifications, if the cladistic method of ranking would be applied.
Mayr (1974g, 1980d) further pointed out difficulties (a) of determining the direction
of evolutionary character changes, that is the polarity of primitive–derived
character sequences, (b) of discriminating between parallelism and convergence
(often ignored by cladists), and (c) of mosaic evolution, that is unequal rates of
evolution of different characters. He felt that evolutionary classifications are more
meaningful biologically, because they pay attention to major adaptive events in
evolutionary history which, according to Mayr, are of greater importance for the
ranking of taxa than the mere splitting of phyletic lines. Evolutionary classifications
permit the greatest number of broad generalizations. Mayr (1981a) proposed
a synthesis in the sense of a first ordering of the data as in phenetics followed
by an analysis of the hierarchical structure buried in the phenetic clusters based
on cladistics and, third, application of evolutionary systematics to produce a mature
classification. At that time Mayr (l.c.) thought it would be possible to convert
a cladogram into a traditional classification by cutting up a clade into segments of
approximately equal degree of difference, that is by converting it into a phylogram.
He realized, however, that this cannot be done successfully and that paraphyletic
groups cannot be properly indicated (Mayr 1995b). Mayr and Bock (2002h) discussed
“Classifications and other ordering systems” from a broad perspective
distinguishing between an evolutionary classification and a cladistic cladification.
The procedure of classifying is to “make homogeneous classes of similar entities”
and the finished product is a “hierarchically ordered system of classes of similar
objects” (p. 181). Ordering systems not based on classes, such as sequential listings
or cladifications (Mayr 1995b), are not classifications. In W. Hennig’s (1966)
system all species are assigned to branches (clades or cladons) of the cladogram
rather than to classes of similar taxa as in the traditional classification. A clade is
holophyletic when it includes the originating stem species and all of its descendants.
Hennig’s term “paraphyly” does not exist in evolutionary classifications,
because the appearance of a new side branch has no retroactive influence on the
classification of its ancestors. Even though birds and mammals budded off from
the Reptila, this ancestral group remains untouched and as sound an evolutionary
taxon as ever, according to Mayr and Bock (l.c.). They emphasized that Hennig
(l.c.) was the first taxonomist to have formally introduced the important concept of
using apomorphous (derived) characters to determine the branching points and
to construct a phylogenetic diagram (cladogram). Cladistic analysis is as useful in
evolutionary classification as it is in Hennigian cladification (but see Bock 1992b).
A sound evolutionary classification must be based on a balanced consideration
of both cladogenesis (genealogical branching) and anagenesis (similarity