Syst. Biol. 45(2):174^189, 1996
EXPLICITNESS, STABILITY, AND UNIVERSALITY IN THE
PHYLOGENETIC DEFINITION AND USAGE OF TAXON NAMES:
A CASE STUDY OF THE PHYLOGENETIC TAXONOMY OF THE
CARNIVORA (MAMMALIA)
HAROLD N. BRYANT
Provincial Museum of Alberta, 12845 102 Avenue, Edmonton, Alberta T5N 0M6, Canada;
E-mail: [email protected]
Abstract.—In phylogenetic taxonomy, the clade denoted by a taxon name is determined by the
phylogenetic definition of the name and by the phylogenetic context. The clade referred to by the
phylogenetic definition of a taxon name depends on (1) the reference phylogeny, (2) the meanings
of taxon names referred to in the definition, and (3) whether the name is defined as a node-, stem-,
or apomorphy-based taxon. Ambiguity in the clade referred to by the definition results from (1)
imprecise wording so that no single ancestor is specified, the clade identified by the definition
changes with the extinction of taxa, or the definition is a tautology, (2) failure to specify a reference
phylogeny, and (3) the lack of phylogenetic definitions of taxon names referred to in the definition.
In phylogenetic taxonomy, phylogenetic pattern has the central role in determining the identity
and attributes of the clades denoted by particular taxon names. Because phylogenetic definitions
may associate taxon names with different clades on different cladograms, definitions may have
to be emended with changes in our understanding of phylogeny to preserve the association between a taxon name and a particular clade. As phylogenetic hypotheses stabilize, so will the
definitions and usage of taxon names. Conventions can promote universality in the formation,
definition, and usage of taxon names in phylogenetic taxonomy. Ambiguity is reduced by using
standard formats for node- and stem-based definitions and by citing authorities for the meaning of
taxon names used in definitions. Reference to undefined taxon names in definitions can be avoided
by referring instead to the species referred to that taxon in a standard taxonomic reference. Recommended usages of taxon names provide a mechanism for restricting the use of a name to a clade
with a particular exclusive common ancestry. Continuity of usage should help to promote the acceptance of phylogenetic taxonomy by biologists. These issues are discussed within the context of
the phylogenetic taxonomy of the Carnivora. [Carnivora; nomenclatural stability; phylogenetic definition; phylogenetic taxonomy; taxon names; taxonomic conventions; taxonomic usage.]
Codes of biological nomenclature include rules and recommendations for the
formation and usage of taxon names. Given that a primary goal of nomenclature is
effective communication among biologists,
the meanings of taxon names should be explicit (thus preventing ambiguity), stable,
and universal (de Queiroz and Gauthier,
1992, 1994, and references therein). The
current codes (International Commission
on Zoological Nomenclature, 1985; International Botanical Congress, 1988; International Association of Microbiological Societies, 1992) attempt to achieve these goals
by tying the formation and usage of taxon
names to the categories in the Linnean hierarchy and to name-bearing types. However, given that (1) the principle of common descent is the fundamental paradigm
of modern biology and (2) taxa should be
historical entities (species, clades), the nonevolutionary nature of the Linnean hierarchy impedes the realization of the abovestated goals (de Queiroz and Gauthier,
1994). Although there is an increasing realization that taxa should be historical entities, most taxonomists continue to work
within this nonevolutionary nomenclatural
system. Recently, de Queiroz and Gauthier
(1990, 1992, 1994) proposed and outlined
basic principles of an alternative system,
phylogenetic taxonomy, in which rules of
nomenclature are based directly on the
tenet of common descent,
Phylogenetic taxonomy is that part of
phylogenetic systematics that deals with
the representation of phylogenetic relationships using a system of names (de Queiroz
and Gauthier, 1992). The names of clades
are defined phylogenetically through ref-
174
1996
BRYANT—PHYLOGENETIC TAXONOMY OF THE CARNIVORA
erence to a common ancestor and all of its
descendants (Ghiselin, 1984); in other
words, "taxon names are names of clades"
(de Queiroz and Gauthier, 1992:452). Phylogeny, not name-bearing types and Linnean categories, provides the frame of reference for determining the taxon to which
a name applies. Linnean ranks contribute
nothing to the representation of phylogenetic relationships and can be omitted (de
Queiroz and Gauthier, 1992). Various recent studies include phylogenetic definitions of taxon names (e.g., Gauthier, 1986;
de Queiroz, 1987; Gauthier et al, 1988;
Rowe, 1988; Sereno, 1991; Ford and Cannatella, 1993; Padian and May, 1993; Wolsan, 1993; Wyss and Flynn, 1993; Demere,
1994; Holtz, 1994; Sundberg and Pleijel,
1994; Bryant, 1996).
The introduction of a new nomenclatural system is inherently destabilizing.
Nonetheless, de Queiroz and Gauthier
(1990,1992,1994) argued that phylogenetic
taxonomy stabilizes the meaning and usage of taxon names through the explicit
reference to a common ancestor and all of
its descendants. Although taxon names
can be defined phylogenetically in the absence of a known phylogeny, phylogenetic
pattern determines not only the content
and diagnostic synapomorphies but also
the identity of the clades associated with
those names. This central role of reference
phylogenies, which may be poorly resolved or in a state of flux, in the interpretation of phylogenetic definitions might be
perceived as a source of instability. Also,
ambiguity is introduced if definitions are
imprecise or ancillary information is lacking. General acceptance of phylogenetic
taxonomy will probably depend on the degree to which confusion and changes in
the usage of taxon names can be minimized. The importance of continuity of usage is illustrated by increasing demands
for taxonomic stability (e.g., Savage, 1990;
Stace, 1991). Ambiguity in the wording of
phylogenetic definitions, stability in the
clades denoted by taxon names, and universality and continuity in the usage of
those names are discussed here within the
context of two phylogenetic taxonomies of
175
the Carnivora (Wolsan, 1993; Wyss and
Flynn, 1993) that included different phylogenetic definitions of various taxon
names and were based on different phylogenies. Explicitness, stability, and universality in phylogenetic taxonomy will require a series of conventions.
Two PHYLOGENETIC TAXONOMIES OF
CARNWORA
Wyss and Flynn (1993) reassessed phylogenetic relationships among major taxa
within the Carnivora (Fig. la) and provided the first phylogenetic definitions of the
names of selected clades inferred through
that analysis (Table 1). The name Carnivora was defined as belonging to a crown
clade that, based on their cladogram, excluded the early Tertiary "Miacidae" and
"Viverravidae," which have been considered members of the Carnivora in most
classifications since Simpson (1945). They
introduced the name Carnivoramorpha for
the more inclusive stem-based taxon that
includes these closest relatives of the
crown clade.
Subsequently, Wolsan (1993) assessed
the phylogenetic relationships of various
European fossil arctoid carnivorans, most
of which have been considered most closely related to procyonids and mustelids.
The explicit phylogenetic analysis considered relationships among only Ailurus and
14 fossil "mustelidan" (see Table 1) genera.
Wolsan provided phylogenetic definitions
for various carnivoran taxon names (Table
1), including six names that had been defined previously by Wyss and Flynn (1993)
and one new name (Arctomorpha). Many
of the names defined previously by Wyss
and Flynn were defined differently, and
most of the taxon names that Wolsan defined do not represent explicit clades
based on his or any other cited phylogenetic analysis. The more inclusive clades
on Wolsan's "phylogeny" (Fig. lb) had to
be inferred from his phylogenetic definitions (Table 1). For example, the Amphicyonidae was not included in Wolsan's
phylogenetic analysis; nonetheless, his
phylogenetic definitions of Arctomorpha
and Arctoidea indicate that he considered
176
SYSTEMATIC BIOLOGY
Arctoidea
k.FeliformiaN/Caniformia
Carnivora
(a)
(b)
Carnivoramorpha
/ / / / / / / / / / /
Carnivora
Jrsida IX Mustelida
{Arctomorpha
(Arctoidea
Feliformia VCaniformia
.Carnivora
/ / / / / /
c> & /a *^ /<$>/ $
Mustelida=Arctomorpha
Arctoidea
Caniformia
(O
VOL. 45
(d)
Ursoidea=Ursida
=Arctoidea
Caniformia
Feliformia
arnivora
r
Carnivoramorpha
FIGURE 1. Phylogenetic relationships and naming of clades within Carnivora as proposed by Wyss and
Flynn (1993) and Wolsan (1993). "Miacidae" and "Viverravidae" occur only on Wyss and Flynn's cladogram,
whereas Ailuropoda occurs only on Wolsan's cladogram. t = extinct taxon. Taxa that are probably paraphyletic
are enclosed in quotation marks. The presence of a line placed directly above a node indicates that the name
refers to a stem-based taxon. (a) Wyss and Flynn's phylogeny and phylogenetic definitions. Taxa included in
their analysis that are not directly pertinent to the present discussion are omitted, (b) Wolsan's phylogeny and
phylogenetic definitions. Only the polytomy consisting of Ailurus, Pinnipedia, Procyonidae, and Mustelidae is
based directly on Wolsan's phylogenetic analysis; other relationships are inferred from his phylogenetic definitions. Feloids is an informal collective term for felids, viverrids, Nandinia, hyaenids, and herpestids. (c) Wyss
and Flynn's phylogeny and Wolsan's phylogenetic definitions. The name Ursida, and consequently Mustelida
and Arctoidea, can be assigned to a clade only with the additional assumption that Ailuropoda is an ursid. (d)
Wolsan's phylogeny and Wyss and Flynn's phylogenetic definitions.
Amphicyonidae an outgroup to all extant
arctoid carnivorans. Wolsan's stem-based
definition of Carnivora is more consistent
with prevalent recent usage of this name
in that it implicitly includes miacids and
viverravids.
Given a phylogenetic definition, the clade
to which a taxon name refers depends on
(1) the branching pattern of the reference
cladogram, (2) the meaning (=definition) of
taxon names referred to in the definition,
and (3) whether the name is associated with
1996
BRYANT—PHYLOGENETIC TAXONOMY OF THE CARNIVORA
a node-, stem-, or apomorphy-based taxon.
Thus, any differences in the clades specified
by the six taxon names that were defined
by both Wyss and Flynn (1993) and Wolsan
(1993) reflect differences in some combination of their phylogenies and definitions.
Neither Wyss and Flynn nor Wolsan used
apomorphy-based definitions (de Queiroz
and Gauthier, 1990, 1992, 1994).
The definitions of three (Feliformia, Caniformia, Pinnipedia) of the six taxon names
defined by both Wyss and Flynn (1993) and
Wolsan (1993) specify the same clades in
the two schemes (Table 1; Figs, la, lb). In
the case of the taxon name Pinnipedia, both
the phylogeny (exclusive monophyly of
seals, seal lions, and walruses; assumed in
Wolsan's analysis) and the essential wording of the definitions are identical in the
two instances. Equivalency of the definitions of Caniformia, and especially Feliformia, is less obvious because different taxa
are referred to in the definitions. Nonetheless, both phylogenies divide extant carnivorans into two major clades (catlike and
doglike carnivorans) to which these names
refer. Although the taxon names referred to
in the two definitions of Feliformia differ,
the ancestry specified by those taxa is the
same. Wyss and Flynn used Feloidea in
their definition of Feliformia, whereas Wolsan listed felids, viverrids, Nandinia, hyaenids, and herpestids (feloids: Fig. lb); the definitions are equivalent because Wyss and
Flynn based their definition of Feloidea on
those same taxa.
In contrast, the definitions of the other
three taxon names (Carnivora, Ursida,
Arctoidea) refer to different clades in the
two schemes (Table 1; Figs, la, lb). All
three taxon names had been defined as
node-based taxa by Wyss and Flynn (1993)
but were redefined as stem-based taxa by
Wolsan (1993). This is the only difference
in the clades associated with the names
Carnivora and Arctoidea; other aspects of
the exclusive common ancestry associated
with the clades are the same. Both definitions of Arctoidea associate this name with
a clade of noncanid caniforms, but because
of differences in relationships within this
clade, the taxon names upon which the def-
177
initions are based differ. Wyss and Flynn
defined Arctoidea based on the most recent common ancestry of procyonids, Ailurus, Ursida, and mustelids. The extinct
Amphicyonidae is part of their Arctoidea
because of its sister relationship to Ursidae
(Fig. la). In contrast, Wolsan considered
Amphicyonidae as the outgroup to the
other taxa included in Arctoidea (Fig. lb).
Nonetheless, Wolsan explicitly included
Amphicyonidae by using this name in his
definition of Arctoidea (Table 1). Wolsan's
use of different taxon names in his definition maintains the association of the name
with noncanid caniforms despite differences in phylogeny within the clade. In
contrast, differences in the clades associated with the taxon name Ursida also involve differences in phylogeny and the taxon names included in the definitions.
Wolsan defined Ursida as a stem-based
taxon based on the common ancestry of
ursids and Ailuropoda, whereas Wyss and
Flynn defined Ursida as a node-based taxon based on the ancestry of ursoids (ursids
and amphicyonids) and pinnipeds.
Wolsan's (1993) redefinition of taxon
names that were defined previously by
Wyss and Flynn (1993) so that they referred to different clades has a destabilizing effect on the meaning and usage of
those names. The different usage of the
names Carnivora and Arctoidea is not associated with differences in phylogenetic
hypotheses but rather reflects the decision
to use these names for stem-based rather
than node-based clades. Given the goal of
nomenclatural stability, such seemingly arbitrary changes in the definition and usage
of taxon names should be avoided. However, given that Wolsan's definition of Carnivora more closely follows recent usage,
the appropriate choice between Wolsan's
and Wyss and Flynn's definitions of this
taxon name might not seem obvious.
AMBIGUITY
Ambiguity in the clade referred to by
the definition of a taxon name results from
(1) imprecise wording of the definition, (2)
failure to specify a reference phylogeny,
and (3) the absence of phylogenetic defi-
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VOL. 45
SYSTEMATIC BIOLOGY
TABLE 1. Comparison of the phylogenetic definitions of Wyss and Flynn (1993) and Wolsan (1993). Definitions are quotes. N = node-based definition; S = stem-based definition.
Taxon name
Wyss and Flynn (1993)
Carnivoramorpha
Carnivora plus all forms more closely
related to it than to Creodonta (S)
the most recent common ancestor of living carnivorans and all of its descendants (N)
not defined
Feliformia
the most recent common ancestor of the
Feloidea and all of its descendants
plus all carnivorans more closely related to feloids than to caniforms (S)
the most recent common ancestral species of felids, viverrids, Nandinia,
hyaenids, and herpestids, and all of
its descendants, plus all carnivorans
that share an ancestral species with
this clade, being not also common to
the Caniformia (S)
Feloidea
not defined
Caniformia
the most recent common ancestor of felids, viverrids (including Nandinia),
herpestids, and hyaenids and all of its
descendants (N)
the most recent common ancestor of
canids and arctoids and all of its descendants plus all carnivorans sharing
an ancestor with this clade that is not
also common to feliforms (S)
Canoidea
not defined
Arctoidea
the most recent common ancestor of
procyonids, Ailurus, Ursida, and mustelids, plus all of its descendants (N)
the most recent common ancestral species of canids and all of its descendants, plus all caniforms that share an
ancestral species with this clade, being
not also common to the Arctoidea (S)
the most recent common ancestral species of amphicyonids and arctomorphs
and all of its descendants, plus all arctoids that share an ancestral species
with this clade, being not also common to the Canoidea (S)
Arctomorpha
not defined
the most recent common ancestral species of ursidans and mustelidans and
all of its descendants, plus all arctomorphs that share an ancestral species
with this clade, being not also common to the Amphicyonidae (S)
Ursida
the common ancestor of ursoids and
pinnipeds ab (N)
the most recent common ancestral species of ursids and Ailuropoda and all of
its descendants, plus all arctomorphs
that share an ancestral species with
this clade, being not also common to
the Mustelida (S)
Ursoidea
the common ancestor of ursids and amphicyonids plus all of its descendants3
(N)
the common ancestor of otariids, odobenids, and phocids plus all of its descendants3 (N)
not defined
Carnivora
Pinnipedia
Wolsan (1993)
the most recent common ancestral species of feliforms and caniforms and all
of its descendants, plus all carnivorans
that share an ancestral species with
this clade, being not also common to
the Creodonta (S)
the most recent common ancestral species of canoids and arctoids and all of
its descendants, plus all caniforms
that share an ancestral species with
this clade, being not also common to
the Feliformia (S)
the most recent common ancestral species of otariids, odobenids, and phocids, plus all of its descendants (N)
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BRYANT—PHYLOGENETIC TAXONOMY OF THE CARNIVORA
TABLE 1.
Taxon name
Continued.
Wyss and Flynn (1993)
Mustelida
not defined
Mustelidae
not defined
Mustelinae
not defined
179
Wolsan (1993)
the most recent common ancestral species of Ailurus, pinnipeds, procyonids,
and mustelids and all of its descendants, plus all mustelidans that share
an ancestral species with this clade,
being not also common to the Ursida
(S)
the most recent common ancestral species of Bassariscus, mephitines, lutrines,
melines, and mustelines (including
mellivorines) and all of its descendants (N)
the most recent common ancestral species of Eira, Galictis, Gulo, Ictonyx, Lyncodon, Maries, Melliwra, Mustela, Poecilictis, Poecibgale, and Vormela and all of
its descendants (N)
' The failure to include "most recent" is probably a simple error of omission.
b
The failure to include "plus all of its descendants" is probably a simple error of omission.
nitions of the taxon names included in the
definition.
Imprecise Wording of Phylogenetic Definitions
The wording of various of the phylogenetic definitions of Wyss and Flynn (1993)
and Wolsan (1993) is ambiguous for one or
more of the following reasons: (1) failure
to clearly specify a single common ancestor for the clade, (2) the clade referred to
is subject to change with the extinction of
taxa, and (3) the wording is tautological.
Wyss and Flynn (1993:38) failed to identify one particular ancestor in defining Pinnipedia as "the common ancestor of otariids, odobenids, and phocids plus all of its
descendants." The taxa listed share many
common ancestors; the definition fails to
identify a single point of common ancestry
by the failure to include the words "most
recent." This ambiguity also occurs in their
definitions of Ursida and Ursoidea and in
some of the definitions of Sereno (1991) and
Ford and Cannatella (1993).
The exclusive common ancestry of a clade
is not altered by its extinction or the extinction of some of its members; the clade
designated by a taxon name must not
change with such events. This change in
designation can occur, however, if words
such as living or extant are used to qualify
taxon names listed in phylogenetic definitions (Lucas, 1992; Bryant, 1994). Wyss and
Flynn (1993:38) defined Carnivora as "the
most recent common ancestor of living
carnivorans and all its descendants." This
definition denotes a clade only if one or
more living carnivorans remain extant,
and the specified common ancestor could
change depending on which species became extinct. Qualifiers that could change
the exclusive common ancestry of the clade
given particular evolutionary events
should be avoided in the wording of phylogenetic definitions.
Two types of tautology occur in the phylogenetic definitions of Wyss and Flynn
(1993) and Wolsan (1993). Tautology occurs
in a phylogenetic definition that refers to the
taxon name that is being defined in the definition. Tautology may also involve pairs of
definitions in which the meaning of each definition depends on the meaning of the other.
The most recent common ancestor stipulated in the phylogenetic definition of a
taxon name depends on the meaning (definition) of taxon names referred to in the
definition. To avoid tautology, a phylogenetic definition must not refer to the name
that is being defined. Wyss and Flynn
(1993:38) defined Carnivora as "the most
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SYSTEMATIC BIOLOGY
recent common ancestor of living carnivorans and all its descendants." However,
"carnivoran" means "member of Carnivora" (Van Valen, 1969), making this definition a tautology. Taxon names must be
defined by referring to other clades or species. For example, Carnivora can be defined as the most recent common ancestor
of Feloidea and Arctoidea and all of its descendants. Wolsan's (1993) definitions of
Carnivora, Caniformia, Arctoidea, Arctomorpha, and Mustelida are also tautologies because they refer to the name that is
being defined.
Although one stem-based phylogenetic
definition of a taxon name may not be a
tautology, two such definitions may be tautologous in combination. This situation occurs in both Wyss and Flynn's (1993) and
Wolsan's (1993) stem-based definitions of
Feliformia and Caniformia (Table 1). Both
definitions of Feliformia make reference to
Caniformia, thus making the definition of
the former dependent on the definition of
the latter. However, both definitions of
Caniformia similarly make reference to
Feliformia, and thus the meaning of Caniformia is equally dependent on that of Feliformia. Together the two definitions form
a tautology. Wolsan's definitions of Canoidea and Arctoidea and of Ursida and Mustelida also form tautologies. This type of
tautology, which involves the definitions of
pairs of names of stem-based taxa, can be
avoided by referring not to the name of the
stem-based sister taxon but rather to that of
a less inclusive clade. Thus, Caniformia
could be defined as Canidae and Arctoidea
and all carnivorans that are more closely
related to these taxa than to Feloidea.
Role of the Reference Phytogeny
Phylogenetic taxonomy differs from traditional approaches in that phylogenetic
pattern has the central role in determining
the attributes and the identity of the clades
to which particular taxon names refer. Given that a clade is a "group of species sharing an exclusive common ancestry" (de
Queiroz and Gauthier, 1994:27), the three
taxa Feloidea, Canidae, and Arctoidea
form the same clade in Figures 2a and 2b
VOL. 45
FIGURE 2. Two possible phylogenetic patterns
among the three major clades of extant carnivorans.
because they constitute the same system of
exclusive common ancestry; however, the
relationships within this clade differ in the
two cladograms. The definition "the most
recent common ancestor of Feloidea and
Arctoidea and all of its descendants" refers
to this entire clade if it is interpreted within the context of Figure 2a. However, if this
definition is interpreted within the context
of Figure 2b, Canidae is excluded despite
the fact that these three taxa still form the
same clade. The definition becomes associated with the less inclusive clade that includes only Feloidea and Arctoidea because the definition was based on the
ancestry of only these two taxa. Phylogenetic definitions associate taxon names
with particular clades only within the context of a reference phylogeny; taxon names
may refer to different clades on different
cladograms. Given this interpretation of
how particular clades are identified, the
statement "under a phylogenetic system of
nomenclature, names retain their associations with particular clades or ancestors
despite changes in ideas about relationships" (de Queiroz and Gauthier, 1994:30)
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BRYANT—PHYLOGENETIC TAXONOMY OF THE CARNIVORA
is incorrect. Because of this central role of
phylogenetic pattern, the failure to specify
a reference phylogeny can result in marked
ambiguity in the interpretation of a phylogenetic definition, both in terms of the
particular clade it denotes and the attributes (e.g., content, diagnostic synapomorphies, time of origin) of that clade.
All of the taxon names defined phylogenetically by Wyss and Flynn (1993) refer to
clades identified through their phylogenetic
analysis. As a result, the clades with those
names can be identified by referring to their
phylogeny. In contrast, although Wolsan's
(1993) explicit phylogenetic analysis considered relationships within only his Mustelida, he defined various taxon names that refer to more inclusive and nonoverlapping
clades within Carnivora. In fact, none of
Wolsan's definitions in Table 1 refer explicitly to the topology of his cladogram. Thus,
most of the pattern in Figure lb is inferred
instead from his phylogenetic definitions
and can include only taxa referred to specifically in the definitions.
Ambiguity also occurs if the reference
phylogeny lacks one or more of the taxa referred to in the phylogenetic definition, e.g.,
when phylogenetic definitions are interpreted within the context of a different phylogeny that includes a different set of terminal taxa. Wolsan (1993:349) defined
Ursida as "the most recent common ancestral species of ursids and Ailuropoda and all
of its descendants, plus all arctomorphs that
share an ancestral species with this clade,
being not also common to the Mustelida."
Wyss and Flynn (1993) did not include Ailuropoda as a terminal taxon in their analysis. As a result, Ursida cannot be identified
on Wyss and Flynn's cladogram using Wolsan's definition. The problem extends to
Wolsan's definitions of Mustelida and Arctomorpha, which refer to Ursida.
Meaning of Taxon Names Listed in
Definitions
The clade referred to by the phylogenetic
definition of a taxon name is directly dependent on the meaning (definitions) of the
taxon names listed in that definition. Ambiguity regarding the meaning of the
181
names used in the definition will result in
ambiguity in the meaning of the name of
the more inclusive clade that is being defined.
Wyss and Flynn's (1993) definition of
Arctoidea lists Procyonidae, Ailurus, Ursida, and Mustelidae. Of these four taxa,
they defined only Ursida. Ailurus is a
monotypic genus and is therefore equivalent to the species Ailurus fulgens. Although there may be little perceived uncertainty regarding the meaning of
Procyonidae and Mustelidae, Wolsan's
(1993) explicit, unorthodox inclusion of
Bassariscus in Mustelidae rather than Procyonidae (Table 1) indicates that in the absence of phylogenetic definitions of these
names their meaning is ambiguous. Wolsan defined Arctomorpha based on the
most recent common ancestry of Ursida
and Mustelida. Although both of these taxon names are also defined, the meanings
of Ursida and Mustelida are in turn dependent on the meaning of the taxon
names used in those definitions. The
meaning of Ursida depends on the meaning of Ursidae. The meaning of the name
Ursidae might not be considered problematic; however, Wolsan's explicit exclusion of
Ailuropoda melanoleuca, which has been con-
sidered an ursid by most recent authors
(e.g., Davis, 1964; O'Brien et al, 1985; Nowak, 1991; Wilson and Reeder, 1993), causes uncertainty regarding Wolsan's meaning
of this name. Thus, the meaning of a taxon
name depends on the meaning of the
names listed in its definition and on the
meaning of the names included in the definitions of those listed names, etc., until the
meaning of listed names are based on the
names of species rather than clades. Thus,
to minimize ambiguity the names of all of
these clades and species must be defined.
There are at least two alternatives to using undefined taxon names in phylogenetic definitions. The dependence of the definition of a taxon name on the definitions
of the names of other clades can be avoided by specifying common ancestry using
the names of particular species that are included in those clades rather than the
names of the clades themselves. Thus, Car-
182
SYSTEMATIC BIOLOGY
nivora could be defined as the most recent
common ancestor of Felis catus and Canis
familiaris and all of its descendants. Given
the knowledge that Felis catus is a member
of Feloidea and that Canis familiaris is a
member of Canidae, the relationship between the definition and the phylogeny
(Fig. 1) is unambiguous. Alternatively, one
can refer to the common ancestry of the
membership of taxa with undefined names
as listed in some standard taxonomic reference (e.g., for crown clades within Mammalia: Nowak, 1991; Wilson and Reeder,
1993). Therefore, instead of referring to
Mustelidae, one could refer to those species referred to Mustelidae by Wilson and
Reeder (1993); because the definition refers
to a list of species rather than a taxon
name, Mustelidae does not have to be defined (see Table 2 for other examples).
These alternatives ignore the important issue of how species names should be defined in phylogenetic taxonomy.
STABILITY
Phylogenetic taxonomy is purported to
stabilize the association between clades
and taxon names by defining those names
with respect to common ancestry (de
Queiroz and Gauthier, 1990, 1992, 1994;
Schwenk, 1994). The phylogenetic definition is said to establish the association between a taxon name and a particular clade.
The role of the reference phylogeny in the
interpretation of phylogenetic definitions
indicates, however, that the association between a taxon name and a particular clade
depends on the definition and on the phylogenetic context. Changes in our understanding of phylogeny can associate taxon
names with different clades. This potential
for changes in the names of clades to result
from changes in phylogenetic hypotheses
might be perceived as indicative of inherent nomenclatural instability in phylogenetic taxonomy.
Wyss and Flynn's (1993) and Wolsan's
(1993) phylogenies (Fig. 1) differ primarily
in the position of Procyonidae, Pinnipedia,
and Amphicyonidae. Taxon names that were
defined by referring either directly or indirectly to these taxa may be associated with
VOL. 45
different clades when their definitions are
interpreted within the context of the two
phylogenies. Wyss and Flynn defined Ursoidea based on the common ancestry of ursids and amphicyonids. On their cladogram,
Ursidae and Amphicyonidae are sister taxa
and Ursoidea is a subclade within Arctoidea
(Fig. la). In contrast, Wolsan considered
Amphicyonidae the outgroup of all extant
arctoid carnivorans. Thus, based on Wolsan's phylogeny, the name Ursoidea, as defined by Wyss and Flynn, becomes associated with a much larger clade that includes
all the extant carnivorans that have typically
been included in the Arctoidea (Fig. Id).
Given Wyss and Flynn's (1993) phylogenetic definitions and Wolsan's (1993) phylogeny, Ursoidea, Ursida, and Arctoidea denote
the same clade (Fig. Id) and are therefore
synonyms in phylogenetic taxonomy (de
Queiroz and Gauthier, 1992). Wyss and
Flynn defined Ursida based on the common
ancestry of ursoids and pinnipeds. However,
within the context of Wolsan's phylogeny,
Ursoidea and Pinnipedia are overlapping
taxa; Pinnipedia is part of Ursoidea and, as
a result, Ursida = Ursoidea. Wyss and
Flynn defined Arctoidea as the most recent
common ancestor of Procyonidae, Ailurus,
Ursida, and Mustelidae and all its descendants. Given that Ursida is synonymous
with Ursoidea, Ursida includes the other
taxa listed in the definition of Arctoidea and,
therefore, Arctoidea = Ursida = Ursoidea.
The synonymies result from the more basal
position of Amphicyonidae in Wolsan's phylogeny and the role of that taxon in determining the clade denoted by these three taxon names, either directly (Ursoidea) or
indirectly (Ursida, Arctoidea). Thus, Wyss
and Flynn's (1993) definitions of Ursoidea,
Ursida, and Arctoidea associate these names
with three different clades on their tree (Fig.
la) but associate these names with the same
clade on Wolsan's (1993) tree (Fig. Id).
Which of these three synonyms should
be used for this clade on Wolsan's (1993)
tree? The exclusive common ancestry of
the clade in question is the same as that of
the clade named Arctoidea by Wyss and
Flynn (1993). In other words, noncanid
caniforms constitute a clade on both clado-
1996
BRYANT—PHYLOGENETIC TAXONOMY OF THE CARNIVORA
grams; only the relationships within the
clade differ (Figs, la, Id). In contrast, the
exclusive common ancestry of the clades
named Ursoidea and Ursida is very different on the two cladograms. On Wolsan's
tree, Ursidae and Amphicyonidae do not
share common ancestry to the exclusion of
other carnivorans and Pinnipedia is considered more closely related to Mustelidae
and Procyonidae than to Ursidae. Although Arctoidea is the appropriate name
for this clade, Wyss and Flynn's definition
should be emended to remove the reference to Ursida because the wording is effectively tautologous because of the synonymy of Ursida and Arctoidea.
De Queiroz and Gauthier (1992, 1994)
saw the phylogenetic definition of a taxon
name as a mechanism for permanently
linking that name to a particular clade.
Stability is certainly best served by maintaining the relationship between individual clades and particular taxon names.
Even if their phylogenetic hypothesis is rejected, Wyss and Flynn's (1993) definitions,
which have priority, should be retained
wherever possible. However, because phylogenetic definitions may associate taxon
names with different clades on revised
cladograms, definitions may have to be
emended to preserve the association between taxon names and particular clades.
UNIVERSALITY AND CONTINUITY OF USAGE
Although the role of phylogeny in the
interpretation of phylogenetic definitions
suggests that universality in phylogenetic
taxonomy cannot be fully realized until
there is agreement regarding phylogenetic
relationships, universality in any nomenclatural system also depends on widespread adherence to a set of rules or conventions for the formation and use of taxon
names. The success of phylogenetic taxonomy will depend on the ability of its adherents to convince other systematists of
the theoretical superiority of this approach
over orthodox systems of nomenclature;
however, its general acceptance among biologists may depend more on the pragmatic issue of continuity, the degree to
which changes in nomenclature can be
183
minimized in the conversion to phylogenetic taxonomy. The following conventions
associated with the formation, definition,
and usage of taxon names are endorsed or
proposed with universality and continuity
in mind; some are heuristic devices that
provide hierarchical information associated
with the taxon whose name is being defined and with other taxa. Individual conventions will be in conflict in particular instances; mechanisms of resolution are not
proposed here. There is no claim to completeness or maximum effectiveness, but
widespread adherence to a set of conventions should facilitate the general acceptance of phylogenetic taxonomy. The definitions in Table 2 follow these conventions.
1. Phylogenetic Definitions Should Name
Clades Identified through Phylogenetic
Analysis
Clades should be identified through phylogenetic analysis prior to being named formally using a phylogenetic definition. Wolsan's (1993) definition of Mustelinae (Table
1) implies that the extant mustelids in this
subfamily in traditional classifications (e.g.,
Simpson, 1945) share common ancestry exclusive of that of other mustelids. However,
Wolsan provided no empirical support for
the monophyly of Mustelinae sensu Simpson (1945). In contrast, Bryant et al. (1993)
suggested that Mustelinae sensu Simpson
is probably paraphyletic. Based on the phylogeny of Bryant et al. (1993: fig. 5), Mustelinae as defined by Wolsan would include
all extant mustelids except Melogale. This
was clearly not Wolsan's intent in defining
this taxon name. Phylogenetic definitions
that were not formulated within an explicit
phylogenetic context might be considered
invalid. This approach is consistent with
the view that the naming of a clade is logically secondary to the recognition of its existence (de Queiroz, 1994) and that the
identity of the clade depends on phylogenetic pattern.
2. Phylogenetic Definitions Should Have
Standardized Formats
The ambiguity associated with the
wording of phylogenetic definitions can be
184
VOL. 45
SYSTEMATIC BIOLOGY
TABLE 2. Emendation of phylogenetic definitions of Wyss and Flynn (1993) and Wolsan (1993) based on the
conventions in this paper. Undefined taxon names are replaced by the species or taxa referred to those taxa in
an appropriate taxonomic listing. N = node-based definition; S = stem-based definition.
Taxon name
Emended definition
Comments
Carnivoramorpha
Carnivora and all members of Mammalia (Rowe, 1988) that are more closely
related to Carnivora than to taxa referred to Creodonta by Carroll (1988)
(S)
Carnivora
most recent common ancestor of Feloidea, all species referred to Canidae by
Wilson and Reeder (1993), and Arctoidea and all of its descendants (N)
Feliformia
Feloidea and all members of Carnivora
that are more closely related to Feloidea than to Canoidea (S)
most recent common ancestor of the
species referred to Felidae, Viverridae,
Herpestidae, and Hyaenidae by Wilson and Reeder (1993) and all of its
descendants (N)
Canoidea and all members of Carnivora
that are more closely related to Canoidea than to Feloidea (S)
most recent common ancestor of Arctoidea and the species referred to Canidae by Wilson and Reeder (1993) and
all of its descendants (N)
most recent common ancestor of the
species referred to Procyonidae, Mustelidae, and Ursidae by Wilson and
Reeder (1993), of Ailurus, and of Pinnipedia and all of its descendants (N)
not emended; Wolsan's definition could
be considered invalid
most recent common ancestor of all species referred to Ursidae by Wilson and
Reeder (1993) and of Pinnipedia and
all of its descendants (N)
(1) refers to an authority for the meaning of Mammalia; (2) should be
emended if Creodonta not closely related to Carnivora (Fox and Youzwyshyn, 1994) or the name Creodonta
is abandoned (Polly, 1994)
(1) refers to a crown clade; (2) no qualifiers such as "living" or "extant"; (3)
avoids tautology; (4) refers to the
three crown clades that are usually
considered carnivorans
(1) avoids tautology; (2) stem-based
equivalent of Feloidea
Feloidea
Caniformia
Canoidea
Arctoidea
Arctomorpha
Ursida
Ursoidea
Pinnipedia
Mustelida
most recent common ancestor of all species referred to Ursidae by Wilson and
Reeder (1993) and of taxa referred to
Amphicyonidae by Carroll (1988) and
all of its descendants (N)
most recent common ancestor of all species referred to Otariidae, Phocidae,
and Odobenidae by Wilson and Reeder (1993) and all of its descendants (N)
most recent common ancestor of all species referred to Procyonidae and Mustelidae by Wilson and Reeder (1993)
and of Pinnipedia and all of its descendants (N)
(1) refers to a crown clade; (2) nodebased equivalent of Feliformia
(1) avoids tautology; (2) stem-based
equivalent of Canoidea
(1) refers to a crown clade; (2) nodebased equivalent of Caniformia
(1) refers to a crown clade; (2) avoids
tautology; (3) refers to all crown
clades that are usually considered arctoids
Wolsan's definition not based on an explicit phylogenetic hypothesis
(1) refers to a crown clade; (2) refers to
a single ancestor; (3) avoids tautology;
(4) modified from Tedford's (1976) usage to include Pinnipedia; (5) usage
not recommended on Wolsan's (1993)
cladogram
(1) refers to a single ancestor; (2) usage
not recommended on cladograms
where the clade denoted by this name
includes crown clades in addition to
Ursidae
(1) refers to a crown clade; (2) refers to
a single ancestor
(1) refers to a crown clade; (2) avoids
tautology; (3) modified from Tedford's
(1976) usage to include Pinnipedia; (4)
usage not recommended on Wyss and
Flynn's (1993) cladogram
1996
BRYANT—PHYLOGENETIC TAXONOMY OF THE CARNIVORA
TABLE 2.
Taxon name
Mustelidae
Mustelinae
Continued.
Emended definition
Comments
most recent common ancestor of Mustela
spp. and other species referred to
Mustelidae by Wilson and Reeder
(1993) and all of its descendants (N)
not emended; Wolsan's definition could
be considered invalid
reduced through the adoption of standard
formats for node- and stem-based definitions (see Bryant [1994] for arguments
against the use of apomorphy-based definitions). Various formats would provide
the necessary standardization; the following is one example.
Node-based definitions of taxon names
could take the form "the most recent common ancestor of A and B [and C, D, etc., as
necessary] and all its descendants." This
definition establishes a nested hierarchy between the taxon that is being named and
the taxa listed in the definition. A and B
(and C, D, etc.) are the names of species or
nonoverlapping clades and, through the definition, become parts of the named clade.
Stem-based definitions of taxon names
could take the form "E [and other taxa as
necessary] and all members of G that are
more closely related to E than to F." The
nested relationships described by this definition are more complex than those described by a node-based definition. E and
F are the names of species or nonoverlapping clades. G is the name of a more inclusive taxon that includes E and F. The
clade that is being named is more inclusive
than E (at least conceptually, if not in
known content) and less inclusive than G.
Reference to "members of G" is preferable
to reference to "taxa" or "species" because
it places the taxon within the context of an
appropriate more inclusive taxon. Also,
reference to "taxa" or "species" might be
inaccurate because the stem-based taxon
might include only part of the most recent
ancestral species of E and F (see de Queiroz and Donoghue, 1990).
To minimize ambiguity, authorities for
the meanings of previously defined taxon
185
(1) refers to a crown clade; (2) Wolsan's
definition is rejected because it was
not based on an explicit phylogenetic
hypothesis
only weak evidence for the existence of
a clade for which this name is appropriate (see Bryant et al., 1993)
names used in a definition should be cited
(see definition of Carnivoramorpha, Table
2). Initially, many taxon names that one
will want to refer to in definitions will be
undefined phylogenetically. In these instances ambiguity is avoided by referring
not to the taxon name itself but to the
membership of the taxon as listed in a
well-known taxonomic reference (e.g., Wilson and Reeder, 1993). Because the names
of most less inclusive clades within Carnivora are not yet defined, most definitions
in Table 2 are worded in this manner.
Meng and Wyss (1994:200) introduced a
slightly different format, the stem-modified
node-based definition, in defining Rodentia
as "the clade stemming from the most recent common ancestor of Mus and all Recent mammals more closely related to it
than to Lagomorpha or any other eutherian
'order' (sensu Simpson 1945)." Using this
definition, the authors were able to associate the name Rodentia with a crown clade
in the absence of well-resolved relationships within that clade. However, need for
a different format and reference to Linnean
ranks are avoided by defining Rodentia as
the most recent common ancestor of the
species referred to Rodentia by Wilson and
Reeder (1993) and all of its descendants.
3. Typification of Taxa Is an Optional
Heuristic Device for Standardizing
Hierarchical Relationships among Clades
with Particular Taxon Names
In the Linnean system, taxa below a particular hierarchical level have name-bearing or nomenclatural types. The namebearing type provides an objective frame
of reference for determining the appropriate use of the taxon name. For example,
186
SYSTEMATIC BIOLOGY
Mustela is the type genus of the family
Mustelidae. Although the type concept is
superfluous to phylogenetic taxonomy
(Sundberg and Pleijel, 1994), it is not incompatible with it (de Queiroz and Gauthier, 1992). The hierarchical relationship
between type taxa and more inclusive taxa
is also a heuristic device for forming
names (Mustela provides the word stem for
Mustelidae) and standardizing the hierarchical relationship between the clades for
which these names are used. Thus, in phylogenetic taxonomy the hierarchical relationship can be formalized by using the
name of the type taxon to define the more
inclusive taxon name (de Queiroz and
Gauthier, 1992). Based on the phylogeny of
Bryant et al. (1993: fig. 5), Mustelidae could
be defined as the most recent common ancestor of Mustela and Melogale and all of its
descendants.
4. Emendation of Phylogenetic Definitions Is
Necessary in Particular Instances but Should
Preserve the Association between Taxon
Names and Clades
VOL. 45
taxon name as previously defined becomes
associated with a different clade but the
original clade is still present. For example,
if Carnivora were defined as the most recent common ancestor of Feloidea and
Arctoidea and all of its descendants, on
Figure 2a Canidae is included in that taxon. If that hypothesis is rejected in favor of
the hypothesis depicted in Figure 2b, Canidae is excluded from Carnivora as defined
above despite the fact that these three taxa
still form a clade exclusive of other extant
mammals. In this instance, the definition
of Carnivora should be emended so that all
three clades are still included. This procedure is conceptually consistent with de
Queiroz and Gauthier's (1992,1994) goal of
fixing the association between taxon names
and particular clades. The need for emendation would have been avoided by including all three names in the definition initially so that the name refers to the same
clade regardless of the relationships
among these three taxa.
5. Crown Clades Should Have "Widely
Known Taxon Names" (de Queiroz and
There are at least four instances in which
Gauthier, 1992:469)
the wording of a phylogenetic definition of
a taxon name should be emended. First,
Gauthier (1986), Gauthier et al. (1988,
the definition should be emended to cor- 1989), Rowe (1988), and de Queiroz and
rect or clarify wording. Second, the defi- Gauthier (1992) have argued that widely
nition can be emended to clarify the mean- known taxon names should be used for
ing of included taxon names, e.g., by crown clades. Gauthier et al. (1989) argued
adding a reference for previously defined that this approach promotes informativenames, replacing an undefined taxon name ness and stability because the existence of
with a reference to membership as given crown clades has potentially greater eviin a taxonomic listing, or subsequently re- dential support (hard and soft morphologplacing that listing with the taxon name ical, molecular, and behavioral characters).
once it has been defined phylogenetically. This approach also reflects how these
Third, the definition should be emended to names are typically used by most bioloremove references to inappropriate taxon gists (de Queiroz and Gauthier, 1992) and
names. For example, reference to Ursida in would end the seemingly unresolvable deWyss and Flynn's (1993) definition of Arc- bates among paleontologists regarding
toidea is inappropriate within the context which clades should be given names such
of Wolsan's (1993) phylogeny because Ur- as Mammalia (see Rowe and Gauthier,
sida is a synonym of Arctoidea on that 1992). (For further discussion of this conphylogeny. These emendations should not vention, see Lucas, 1992; Rowe and Gauchange the clade to which the name refers. thier, 1992; Bryant, 1994.)
Fourth, the definition should be emendThe names of crown clades should be
ed to maintain the association between the defined through reference to the names of
taxon name and a particular clade if with other crown clades or extant species. Ala change in the reference phylogeny the though Wyss and Flynn's (1993) definition
1996
BRYANT—PHYLOGENETIC TAXONOMY OF THE CARNIVORA
of Arctoidea (Table 1) denotes a crown clade
on their tree, it does not denote a crown
clade on Wolsan's (1993) tree because the
clade denoted by the name depends indirectly on a fossil taxon (Amphicyonidae).
When names are defined to denote crown
clades, the phylogenetic position of fossil
taxa and convention 4 (preservation of the
association between clades and names) are
ignored. The crown clade definition of Arctoidea in Table 2 denotes different clades
on Wyss and Flynn's and Wolsan's trees
(Fig. 1); Amphicyonidae is included on the
former but not on the latter although it is
part of the same system of exclusive common ancestry (same clade). In Table 2, all
names of node-based taxa except Ursoidea
are defined as crown clades.
6. Given Several "Widely Known Taxon
Names" That Could Be Used for a Crown
Clade, the One at the Lowest Rank in the
Linnean Hierarchy Should Often Be Chosen
In many instances, more than one widely known name could be used for a particular crown clade. For example, either
Equus or Equidae could be used to name
the clade of extant horses. By using Equus
for the crown clade, the familiar name
Equidae remains available for the more inclusive clade that also includes the fossil
taxa that most neontologists and paleontologists have traditionally called horses
(Bryant, 1994).
7. Stem-Based Taxon Names Could Be
Formed from the Name of the Appropriate
Less Inclusive Node-Based Taxon and an
Appropriate Suffix
Given the hierarchical relationship between a node-based taxon and the more
inclusive stem-based taxon, there are heuristic advantages to forming the name of
the stem-based taxon by adding an appropriate suffix to the name of the node-based
taxon (de Queiroz and Gauthier, 1992).
Suffixes such as "-morpha" or "-formia"
seem appropriate, although de Queiroz
and Gauthier (1992) suggested "-gens" or
"-genea," meaning clan. Both "-morpha"
and "-formia" have already been used to
form the names of more inclusive stem-
187
based taxa associated with node-based
taxa such as Mammalia (Mammaliamorpha, Mammaliaformia; Rowe, 1988), Lepidosauria (Lepidosauromorpha; Gauthier et
al., 1988), and Carnivora (Carnivoramorpha; Wyss and Flynn, 1993; Table 1). In
many instances, however, names formed in
this way have been defined as more inclusive node-based rather than stem-based
taxa. Established taxon names that have
these or similar suffixes (e.g., names of
Linnean orders of birds and teleost fishes)
could be defined as stem-based taxa, and
new appropriate names based on the same
word stem could be devised for the equivalent node-based taxa. This procedure
would conflict with convention 5, however.
8. Taxon Names Should Have Recommended
Usages
The usage of taxon names should be restricted to appropriate phylogenetic contexts. Taxon names should be considered
inappropriate if the hierarchical relationships described by the definition do not
occur on the reference cladogram. For example, the use of Mustelida as defined by
Wolsan (1993) is inappropriate within the
context of Wyss and Flynn's (1993) tree because the relationships described by Wolsan's definition (Table 1) do not exist. No
carnivorans are more closely related to Ailurus, pinnipeds, procyonids, and mustelids than to Ursida because the taxa listed
in Wolsan's definition of Ursida share the
same ancestry. In other words, Ursida is
part of Mustelida, and the ancestry of
Mustelida is node based (Fig. lc) despite
its stem-based definition. Wyss and Flynn
did not define Mustelida, possibly because
no clade with an exclusive common ancestry resembling the clade to which this
name has been applied historically (e.g.,
Tedford, 1976) occurs on their cladogram.
Phylogenetic definitions could be accompanied by a stated recommended usage that is designed to limit the use of the
name to particular phylogenetic contexts.
Recommended usages should reflect the
exclusive common ancestry upon which
the definition was based and should consider historical usage. Traditionally, the
188
VOL. 45
SYSTEMATIC BIOLOGY
true seals, walruses, and sea lions have
been grouped as a taxon called Pinnipedia
(e.g., Simpson, 1945; Nowak, 1991). Recent
studies (e.g., Wyss, 1987; Flynn et al., 1988;
Wyss and Flynn, 1993; Berta and Wyss,
1994) suggest that these carnivorans share
an ancestry exclusive of that of other extant carnivorans, overturning the view
(e.g., Tedford, 1976) that as a group these
aquatic carnivorans are diphyletic. The
name Pinnipedia has been defined accordingly (Tables 1, 2). On cladograms congruent with the diphyly hypothesis, this definition would refer the name Pinnipedia to
a more inclusive clade that might include
most or all extant arctoids. Pinnipedia is
an inappropriate name for such a clade because of the historical use of this name and
because of the marked change that would
occur to the exclusive common ancestry of
the clade with this name. Synonymy cannot solve the problem if the more inclusive
clade lacks a previously defined name. The
definition of Pinnipedia should stand, but
recommended usage might restrict its application to hypotheses in which sea lions,
true seals, and walruses share common ancestry exclusive of that of other carnivorans. Recommended usages of names for
crown clades would ignore the phylogenetic position of fossils because the relationship between the name and the clade
is based solely on the relationships among
extant taxa. Recommended usage could
also be a basis for choosing among synonyms (e.g., Arctoidea, Ursida, and Ursoidea in Fig. Id). Recommended usages
will not reduce freedom of taxonomic
opinion but should increase universality
by influencing the usage of taxon names
(see Table 2 for additional examples).
about phylogenetic pattern. Phylogenetic
definitions and taxonomies will stabilize as
phylogenietic hypotheses, or portions
thereof, become more strongly corroborated and stabilized. This relationship is appropriate because it reflects the fact that
phylogenetic taxonomy is based on the
tenet of common descent.
ACKNOWLEDGMENTS
I thank K. de Queiroz, J. Gauthier, Jin Meng, and
A. Wyss for discussion of aspects of phylogenetic taxonomy. K. de Queiroz and two anonymous reviewers
commented on earlier versions of this paper.
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