Species, Taxonomy, and Systematics 417alternative systems have their own problems [Mayr, 1969; Wiley, 1981; Forey,
2002; Brummit, 2002]. This section introduces the problems facing the Linnaean
hierarchy as well as alternatives to that system.
4.1 Linnaean Ranks
Linnaeus’s original hierarchy consisted of five ranks: subspecies (variety), species,
genus, order and class. Taxonomists in the early 20thCentury found Linnaeus’s
5 ranks insufficient for representing life’s diversity, so they increased the number
of ranks to 21. A persistent question concerning the Linnaean ranks is how to
define them. We have seen that biologists and philosophers spend a considerable
amount of time defining the species category. What of the other Linnaean ranks?
How are they defined? Two schools of taxonomy dominated 20thCentury biology:
Evolutionary Taxonomy and Cladism. Biologist in each school proposed different
definitions of the higher Linnaean ranks (those ranks above the rank of species).
However, none of those definitions have withstood criticism.
Before getting to those definitions we need a quick introduction to Evolution-
ary Taxonomy and Cladism. Evolutionary Taxonomy was founded by Ernst Mayr
[1969] and Gaylord Simpson [1961]. Evolutionary taxonomists attempt to capture
two types of phenomena in taxa. First, a taxon is a single genealogical entity. Sec-
ond, the members of a taxon inhabit a common adaptive zone and share a common
way of life. Birds, for example, form a taxon for evolutionary taxonomists because
birds are a single genealogical entity, and birds, for the most part, share a common
adaptive zone that causes them to have a relatively distinct way of life. The other
major taxonomic school of the 20thCentury, Cladism, was founded by Willi Hen-
nig [1966]. Cladists attempt to capture only one type of biological phenomena —
genealogy. For cladists, a taxon contains all and only the descendents of a common
ancestor. Such taxa are monophyletic. Cladists do not recognize birds as a taxon
because doing so would make reptilia a non-monophyletic taxon. Crocodiles are
more closely related to birds than to other reptiles, yet crocodiles are recognized
as part of reptilia, see Figure 4.1. If birds are recognized as a taxon separate
from reptilia, then reptilia would not be monophyletic — some of the reptilia’s
descendants, birds, would not be included in reptilia. Only genealogy matters for
cladists, and removing birds from reptilia has no genealogical justification.
Because evolutionary taxonomists and cladists disagree on what counts as a
taxon, they disagree on how to define the higher Linnaean ranks. For the evolu-
tionary taxonomist, the higher Linnaean ranks are defined in terms of phenotypic
diversity and ecological breadth: the greater the phenotypic diversity and ecolog-
ical breadth of a taxon, the more inclusive the taxon. All families, for example,
contain a certain degree of phenotypic diversity; and, all families occupy an adap-
tive zone of a certain width. The adaptive zone of a family will be smaller than
the adaptive zone of a tribe, which is why, according to evolutionary taxonomists,
families are less inclusive taxa than tribes.
The concepts of ‘phenotypic diversity’ and ‘adaptive zone’ were widely used