3 The New York Years (1931–1953)

Classification 323
Fig.10.1. Cladogram of taxa A, B, and C. Cladists combine B and C into a single taxon,
because B and C share the synapomorph character b. Evolutionary taxonomists separate C
from A and B, which they combine, because C differs by many (c through k) autapomorph
characters from A and B and shares only one (b) synapomorph character with B (from Mayr
1981a)
morphological or nonmorphological characters as well as between-group character
gaps. Evolutionary classification is “horizontal,” as it stresses grouping together
species in a similar stage of evolution rather than location on the same phyletic
line. On the other hand, a cladistic classification is “vertical,” because it stresses
common descent and tends to unite ancestral and descendent groups of a phyletic
line in a single higher taxon (“clade”), separating them from closely related contemporaneous
taxa having reached a similar grade of evolutionary change.
After Simpson’s (1961) text, the 1969(b) edition is still widely used, even by
cladists whose basic philosophy is quite different from Mayr’s (and Simpson’s) approach.
Earlier books were those of Rensch (1934) and Mayr et al. (1953a). To the
1991(a) edition of Principles of Systematic Zoology P. Ashlock mainly contributed
to the section on macrotaxonomy including the chapter on numerical phenetics.
Because his coauthor soon fell seriously ill, most of the work again remained with
Mayr. The chapter on nomenclature is now mostly a critical commentary. The volume
continues to be an excellent introduction to systematic zoology. It emphasizes
that Darwin had actually stressed the importance of phylogeny for classification.
Ever since then systematists have shown that genealogies are essential for studying
adaptation, coevolution, macroevolution and historical biogeography. Also ecologists,
behaviorists, molecular and developmental biologists have successfully used
phylogenetic methods in their research.
Mayr (1974g, 1981a, 1986i, 1995b; Mayr and Ashlock 1991a; Mayr and Bock
2002h) published a series of discussions of ordering systems in zoology. The first
(1974g) contrasts cladistic analysis with cladistic classification (both introduced
by the German entomologist Willi Hennig 1950, 1966 who designated them phy-