Syst. Biol. 47(4):719–726,1998
Phylogenetic Uncertainty, Molecular Sequences, and the DeŽnition
of Taxon Names
MICHAEL S. Y. LEE
School of Biological Sciences, University of Sydney, Sydney, New South Wales 2006, Australia
One of the most important recent developments in taxonomy has been the increasing
acceptance that biological taxa should be deŽned in terms of phylogenetic relationships,
rather than essential characters (de Queiroz,
1994). Three types of phylogenetic deŽnitions of taxon names have been proposed:
Node-based. The most recent common ancestor of A
and B, and all its descendants. The least inclusive
clade containing A and B.
Stem-based. All taxa that share a more recent ancestor
with A than with taxon B. The most inclusive clade
containing A but not B.
Apomorphy-based. The Žrst ancestor with trait X, and
all its descendants. The clade diagnosed by trait X.
In each case the original formulation (de
Queiroz and Gauthier, 1990, 1992) is presented, along with the recently proposed
modiŽed phrasing (Schander and Thollesson, 1995) that removes reference to ancestors. The latter reformulations blunt the
criticism that phylogenetic deŽnitions are
problematic because they implicate the “discarded concept” of ancestors (Dominguez
and Wheeler, 1997:372).
The use of such phylogenetic deŽnitions
is becoming increasingly prevalent, especially in studies dealing with higher vertebrates (e.g., Estes et al., 1988; Gauthier et
al., 1988; Rowe, 1988; Sereno, 1991; Ford
and Cannatella, 1993; Padian and May, 1993;
Wolsan, 1993; Wyss and Flynn, 1993; Holtz,
1994, 1996; Meng and Wyss, 1994; Laurin
and Reisz, 1995; Lee, 1998; Lee and Caldwell, 1998; Dilkes, 1998) and has generated considerable debate (e.g., Lucas, 1992;
Rowe and Gauthier, 1992; Bryant, 1994,1996,
1997; Sundberg and Pleijel, 1994; Schander and Thollesson, 1995; Lee, 1996a, b;
Lidén and Oxelman, 1996; Wyss and Meng,
1996; Cantino et al., 1997; Dominguez and
Wheeler, 1997; de Queiroz, 1997). For inDirect correspondence to the author’s current address: Department of Biological Sciences, Monash
University, Clayton, Victoria 3168, Australia; E-mail:
[email protected]
stance, some have suggested that ranksignifying sufŽxes in conventional Linnaean
names convey information about inclusiveness of taxa, whereas names in the phylogenetic system do not (e.g., Lidén and Oxelman, 1996; Dominguez and Wheeler, 1997).
However, rank-signifying sufŽxes can lead
to conict between associating a name with
a particular clade or with a particular Linnaean category (de Queiroz, 1997) and encourage unjustiŽed assumptions about the
“equivalence” of clades sharing the same
sufŽx (Sundberg and Pleijel, 1994; Knox,
1998). Furthermore, Linnaean sufŽxes usually do not convey information about inclusiveness of taxa (Lee, 1996b). Dominguez
and Wheeler (1997) claim, for instance, that
sufŽxes show that Dasycerinae is nested
within Staphylinidae. No such information
is actually conveyed, however; all that can
be acertained from the spelling of the names
is that Dasycerinae is nested within (or possibly redundant with) some taxon ending in
-idae. Without external information such as
a reference cladogram, it is impossible to
infer that the more-inclusive -idae taxon is
Staphylinidae (Lee, 1996b). Nesting of taxa
would be conveyed only in the rare instances
where taxa have the same root but different
Linnaean sufŽxes (e.g., Iguanidae, Iguaninae).
Another area of debate concerns the stability of taxon names with phylogenetic deŽnitions compared with those having conventional deŽnitions based on types, characters,
and Linnaean categories. Some have argued
that phylogenetic deŽnitions of taxon names
are undesirable because the contents of the
named clade can change if accepted phylogenetic relationships change (Lid én and
Oxelman, 1996; Dominguez and Wheeler,
1997). However, despite assertions that phylogenetic taxonomy is “less stable than the
Linnaean system by any meaningful measure” (Dominguez and Wheeler, 1997:367),
these workers have not yet attempted to
719
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SYSTEMATIC BIOLOGY
demonstrate how a monophyletic Linnaean
system would afford greater stability in such
circumstances (e.g., Lee, 1996b). This is because such instability pervades any monophyletic system of nomenclature (e.g., Nelson, 1973; Gaffney, 1979).
Here, I discuss how such instability might
be minimized in the context of a particular monophyletic system, phylogenetic taxonomy. Previous writers have not emphasized that clades named by the three types
of phylogenetic deŽnitions are not always
equally stable in content, or that one definition is not always superior to the others. If conciseness of deŽnitions is valued,
a node-based deŽnition is preferable when
certain areas of a phylogeny are poorly corroborated, whereas a stem-based deŽnition
is superior if other areas are poorly corroborated. The recommended context for
apomorphy-based deŽnitions, and the relevance of the three types of deŽnitions to
molecular phylogenies, will also be discussed. If these suggestions are correct, the
stability of named clades, and conciseness
of deŽnitions of taxon names, can be improved by knowing when to employ each
type of phylogenetic deŽnition. Conversely,
the practice of applying node-based names
to all clades (e.g., Laurin and Reisz, 1995) is
not optimal but can result in named clades
with unstable contents, cumbersome deŽnitions, or both in many situations. Similarly,
the practice of always applying node-based
names to clades bounded by extant forms
(“crown-clades”) and applying stem-based
names to more-inclusive clades containing
fossil forms, without considering which areas of the reference phylogeny are poorly
corroborated (e.g., Wolsan, 1993), can result
in similar problems (Lee, 1996a; Cantino et
al., 1997).
The preceding approach does place more
emphasis on the (verbal) deŽnitions of
names than on the concepts (clades) described by those deŽnitions (see de Queiroz,
1994). Thus, where a taxon name can be applied to two slightly different clades (e.g.,
a stem-based or a node-based clade) with
the same known contents, the clade that can
be described more succinctly is preferred.
However, if one instead emphasizes the con-
VOL. 47
cept (clade) more than the verbal deŽnition, then attaching a name to a particular
clade has primacy; whether or not the chosen clade can be described succinctly becomes largely irrelevant. If this perspective
is adopted, the recommendations of this paper may not be acceptable. Longer deŽnitions also have advantages, some of which
are discussed at the end of this paper.
Most phylogenetic analyses produce cladograms containing both highly and poorly
corroborated groupings. A typical result is
shown in Figure 1a, where certain clades are
well supported, having Bremer indices of 5
or more and bootstrap frequencies of at least
90%. The other clades are only weakly supported. A conservative cladogram for the
group, with the dubious clades collapsed
into polytomies, is shown in Figure 1b.
I will focus on the properties of stemand node-based phylogenetic deŽnitions
as applied to a particular grouping on the
conservative cladogram (Fig. 1b): the wellcorroborated clade FGHIJ, whose internal
relationships are poorly supported. The
taxon name Zeligidae will be deŽned so
that it includes all Žve taxa, and only these
Žve. (This name refers to Leonard Zelig,
the human doppelganger in the eponymous
Woody Allen Žlm who so desires acceptance that he continually metamorphoses to
resemble those around him, thus creating
difŽculties for those trying to associate his
name with a recognizable biological entity.)
A node-based deŽnition of Zeligidae might
then be, “the least-inclusive clade containing F and J.” If all the poorly corroborated
clades on the most-parsimonious cladogram
(Fig. 1a) are later supported by additional
characters, then this deŽnition would indeed include F, G, H, I, and J, as desired.
However, the contents of Zeligidae as deŽned are highly unstable, given the current
uncertainty regarding relationships among
F, G, H, I, and J. If, based on additional characters, F and J are later identiŽed as closest
relatives (Fig. 1c), then the taxon Zeligidae
would suddenly exclude G, H, and I. The
same uncertainty would apply to any nodebased deŽnition that referred to only two,
three, or four of the Žve taxa. There will
always be the possibility that the taxa men-
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POINTS OF VIEW
721
FIGURE 1. (a) A hypothetical reference phylogeny, with both well-corroborated and poorly corroborated clades.
The taxa to be included in the clade to be named Zeligidae are shown in bold font. For each clade, the Žrst number
is the Bremer index; the second number, the bootstrapping percentage value. (b) A conservative phylogeny, with
poorly corroborated clades in cladogram (a) collapsed into polytomies. (c) A new resolution of the uncertainty in
cladogram (b), where F and J form a clade. A node-based deŽnition of Zeligidae using F and J as reference taxa
differs widely in content between cladogram (a) and the new arrangement. (d) A hypothetical reference phylogeny
with the same topology as cladogram (a) but differing degrees of support for each clade. The taxa to be included
in the clade to be named Zeligidae are shown in bold font. (e) A conservative phylogeny, with poorly corroborated
clades in cladogram (d) collapsed into polytomies. (f) A new resolution of the uncertainty in (e), where C forms
the sister-group to all other taxa. A stem-based deŽnition of Zeligidae that uses C and H as reference taxa differs
widely in content between cladogram (d) and the new arrangement.
tioned in the deŽnition will form a clade to
the exclusion of the remaining taxa, which
would thus be excluded from Zeligidae. The
only way to ensure that a node-based definition of Zeligidae will include F, G, H, I,
and J, regardless of how the uncertainty in
Figure 1b is eventually resolved, is to include all Žve taxa in the deŽnition (Cantino
et al., 1997), which then becomes “the leastinclusive clade containing F, G, H, I, and J.”
However, such a deŽnition is now rather
long and cumbersome, because all Žve included taxa (which in a real situation might
be long binomials) have to be listed. One of
the advantages of phylogenetic deŽnitions
is that taxa can be deŽned by simple reference to clades, rather than by exhaustive
enumeration of “essential” characters or included taxa (e.g., de Queiroz, 1994). This
advantage partly disappears if a complete
list of included forms has to be included in
a phylogenetic deŽnition to make the name
stable.
However, the name Zeligidae can be associated with a clade containing FGHIJ much
more succinctly by using a stem-based definition. The deŽnition “the most-inclusive
clade containing H but not C” will always
include F, G, H, I, and J—and only these taxa.
The uncertainty in Figure 1b might eventually be resolved in one of several ways;
it is very unlikely, however, that the clade
FGHIJ will be dismantled. Thus, pending
a drastic revision of phylogenetic relationships, F, G, I, and J will be more closely related to H than to C. Furthermore, the external relationships of the named clade are
well-corroborated: The nearest outgroup is
the clade CDE, and the monophyly of this
outgroup is well-corroborated. Thus, there is
little possibility of D or E being more closely
related than C to clade FGHIJ and thus of
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SYSTEMATIC BIOLOGY
being inadvertently “captured” by such a
stem-based deŽnition.
Thus, if brevity is desirable, stem-based rather than
node-based deŽnitions should be used if relationships within the named clade are uncertain but the
external relationships are well-corroborated. In these
situations stem-based deŽnitions are unaffected by
uncertain relationships within the named clade.
Stem-modiŽed node-based deŽnitions
have recently been proposed (Wyss and
Meng, 1996): If it is assumed that all organisms in Figure 1b are extant, such a definition of Zeligidae would be “the leastinclusive clade containing H and all living
organisms more closely related to H than to
C.” The phylogenetic relationships invoked
in this type of deŽnition are therefore the
same as those of stem-based deŽnitions.
They should perhaps more appropriately
be called “node-modiŽed stem-based deŽnitions,” since the basic structure of deŽnition is similar to a stem-based deŽnition but
is altered slightly (by the addition of “least
inclusive” and “living”) to apply to a morerestricted clade. In the context of phylogenetic uncertainty, therefore, stem-modiŽed
node-based deŽnitions behave like normal
stem-based deŽnitions and (not surprisingly) are also appropriate for clades with
uncertain basal relationships (see Wyss and
Meng, 1996).
A different situation applies to the example illustrated in Figure 1d–f. The topology
of the phylogeny in Figure 1d is identical
to that in Figure 1a, but support for each
clade is different. A conservative cladogram
for the group, with the dubious clades collapsed into polytomies, is shown in Figure
1e. Again, we wish to attach the taxon name
Zeligidae to a clade including taxa F, G, H,
I, and J, and only these taxa.
Here, a node-based deŽnition is more appropriate. The deŽnition of Zeligidae as “the
least-inclusive clade containing F and J” will
always refer to the clade FGHIJ, regardless of
how the relationships among the other taxa
(A–E) eventually resolve. Because the clades
FG and HIJ are both strongly supported, any
two taxa (one from each clade) can be used to
form a stable and succinct node-based deŽnition that will include taxa F, G, H, I, and J,
and only these taxa.
VOL. 47
In contrast, stem-based deŽnitions in this
situation result in names being attached to
unstable clades, unless the deŽnitions are
made long and cumbersome. The deŽnition
of Zeligidae as “the most-inclusive clade
containing H but not C” will contain only
FGHIJ if the poorly corroborated nodes on
the most-parsimonious cladogram (Figure
1d) are later further supported on the basis of
additional evidence. However, if later studies suggest that any or all of A, B, D, and E are
more closely related to H than is C, then the
former four taxa will be included (inadvertently) in Zeligidae (Fig. 1f). Thus, the only
stem-based deŽnition of Zeligidae that will
be stable in content despite this phylogenetic uncertainty would have to be phrased
as “the most-inclusive clade containing H
but not A, B, C, D, or E.” Any taxon in the
well-corroborated clade FGHIJ can be used
instead of H. However, because the relationships among A, B, C, D, and E to the clade
of interest (FGHIJ) are poorly corroborated,
the only way to guard against inadvertently
“capturing” one or more of them in a stembased deŽnition is to mention and exclude
all of them in the deŽnition. As a result, six
taxa have to be included in the phrasing of
the deŽnition.
Thus, if brevity is desirable, node-based rather than
stem-based deŽnitions should be used if relationships within the named clade are well-corroborated
but external relationships are poorly supported. In
these situations, node-based deŽnitions are unaffected by uncertain external relationships of the
named clade.
As mentioned before, stem-modiŽed
node-based deŽnitions have properties similar to those of stem-based deŽnitions and
would thus also be unstable in this context.
The stability of clades named by apomorphy-based deŽnitions also needs to be discussed. If a unique and unreversed apomorphy diagnoses a well-corroborated clade
(e.g., AB in Fig. 1a), a deŽnition of a name
that uses that apomorphy will (obviously)
refer to a well-corroborated clade and be
stable in content. In particular, that putative apomorphy is almost certainly homologous in all the taxa possessing it, and
the deŽnition will refer to a monophyletic
group. If, on the other hand, a unique and
unreversed apomorphy diagnoses a poorly
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POINTS OF VIEW
corroborated clade (e.g., HIJ in Fig. 1a), a
deŽnition using that apomorphy will refer
to a dubious clade and have unstable contents. There is usually little justiŽcation for
naming such clades in any case. In particular, there is a real possibility that addition
of new taxa and/or characters to the analysis will reveal that the putative apomorphy
is actually convergent in the taxa that possess it, and the deŽnition will refer to a
polyphyletic group (Bryant, 1994; see below). In such cases, the deŽnition will have
to be either abandoned or substantially revised. Stability of a clade named by an
apomorphy-based deŽnition therefore depends on the support provided by other
characters.
Homoplastic apomorphies should be
avoided in deŽnitions, even if one of the convergent acquisitions or reversals helps diagnose a well-corroborated clade. Such clades
should be named with use of other, nonhomoplastic apomorphies. However, certain
types of homoplasy (i.e., convergence) are
more problematic than others (i.e., reversal).
For instance, the deŽnition “the taxon diagnosed by wings” will refer to a polyphyletic
assemblage since wings have evolved convergently in insects, Žsh, pterosaurs, birds,
bats, and plant seeds. However, the deŽnition “the taxon diagnosed by the tetrapod
limb” will still refer to a discrete clade, even
though subsequent reversals have occurred
in snakes, amphisbaenians, caecilians, and
other taxa. Because apomorphy-based phylogenetic deŽnitions refer to the origin of a
trait but not to its subsequent history, the origin of the trait must be a unique and clearly
identiŽable event; the subsequent history of
the trait is less relevant. Use of an apomorphy that exhibits reversals will therefore not
result in the recognition of a paraphyletic
group (contra Bryant, 1994).
Thus, apomorphy-based deŽnitions should only be
used if there is strong evidence that apomorphy has
a single origin, helps diagnose a well-corroborated
clade, and can therefore be conŽdently assumed to
be homologous in all taxa possessing it.
Dominguez and Wheeler (1997) have criticized apomorphy-based deŽnitions for focusing on the origin of a trait rather than
on its subsequent history. Because the di-
723
agnostic character might be lost (absent) in
some members of a named clade, a reference cladogram is required to identify the
contents of that named clade. They propose
an alternative system, which they claim obviates the need for a reference cladogram,
i.e., basing a taxon name on a “deŽning”
character and interpreting organisms with
“subsequent modiŽcations” (including total loss) of that character as somehow still
“possessing” it. However, it is still difŽcult
to see how such deŽnitions can be applied in
the absence of a reference cladogram. Suppose Tetrapoda is deŽned as “all organisms
with four pentadactyl limbs;” without some
cladogram, however, one could not possibly
infer that bird wings and snake “nothings”
are modiŽcations of pentadactyl limbs and
that birds and snakes are thus tetrapods.
Node-based and stem-based deŽnitions
can be framed with reference to molecular phylogenies. However, apomorphybased deŽnitions and traditional characterbased deŽnitions cannot readily accommodate molecular sequence information, although certain genetic traits can be used
(e.g., gene duplications and rearrangements). Any character-based deŽnitions expressed in the context of nucleotide or amino
acid sequences will be extremely cumbersome and never achieve widespread usage.
Node-based and stem-based phylogenetic
deŽnitions therefore have the advantage
that they can readily incorporate the rapidly
burgeoning information from such data. In
particular, novel clades discovered by using such methods (e.g., the hippopotamus–
cetacean clade: Gatesy et al., 1996; Hasegawa
and Adachi, 1996; Buntjer et. al., 1997;
Gatesy, 1997) cannot be deŽned by using apomorphy-based or traditional deŽnitions unless one can also Žnd uniquely derived morphological apomorphies congruent with the molecular results. However, as
such clades were not previously suspected
on the basis of conventional (morphological,
behavioral, etc.) analyses, the search for unambiguous nonmolecular apomorphies to
use in such deŽnitions might be fruitless.
This observation blunts the argument that
deŽnitions of taxon names “should be related to the evidence [= characters] from
724
SYSTEMATIC BIOLOGY
which they are derived” (Dominguez and
Wheeler, 1997:369): In some situations, this
will be highly impractical if not impossible.
Thus, stem-based and node-based deŽnitions are
more widely applicable because, unlike apomorphybased deŽnitions (and traditional character-based
deŽnitions), they can be used to name clades identiŽed by molecular sequence data.
The above recommendations are not hard
rules; rather, they should be considered
along with the suggestions of de Queiroz
and Gauthier (1992), Bryant (1996), and
Cantino et al. (1997). In some cases particular conventions may conict (Cantino et
al. 1997). For instance, de Queiroz and Gauthier (1992) and Bryant (1996) both recommended standardizing widely known taxon
names by giving them node-based, crownclade deŽnitions. However, if basal relationships within a clade to be named are poorly
corroborated (as in Fig. 1b), a node-based
deŽnition will be less stable or concise than
a stem-based deŽnition. Well-known taxon
names, if attached to such clades via nodebased deŽnitions, will therefore have unstable contents or cumbersome deŽnitions
(Lee, 1996a; Cantino et al., 1997). The alternative of attaching well-known taxon names to
such clades by using stem-based deŽnitions
might not be acceptable to some biologists.
Even in a stem-based deŽnition, all known
fossils that fall outside the diversity of the
living clade can be excluded by using as a
reference taxon the nearest known fossil outgroup to the living clade. However, future
work might unearth even closer fossil relatives of the living clade. A well-known taxon
name attached to this living clade through
the use of a stem-based deŽnition, therefore,
would “capture” such new fossils. In this situation, taxonomists trying to deŽne a wellknown taxon name will have to weigh the
stability of the content, and brevity of the
deŽnition, of a stem-based deŽnition against
the greater exclusivity (with respect to future fossil discoveries) of a node-based definition. If the latter approach is preferred, a
long and complex deŽnition would be required. A stem-modiŽed node-based deŽnition (Wyss and Meng, 1996; see above)
might be the most appropriate choice in this
context.
VOL. 47
Bryant (1997) has recently suggested that
“designated phylogenetic contexts” should
be added to deŽnitions of taxon names.
DeŽnitions of taxon names are deemed
valid only in the context of particular reference phylogenies, thus stabilizing their
usage and content. For instance, a deŽnition of Haematothermia might be “the
least-inclusive clade including birds and
mammals.” This name, which was intended
to refer to a bird–mammal clade, would be
valid only in the context of heterodox phylogenies that group birds and mammals as
extant sister groups (e.g., Gardiner, 1982;
Lovtrup, 1985; Gardiner, 1993). However,
this approach does not solve completely the
problem of nomenclatural instability caused
by uncertain phylogenetic relationships.
Under previous schemes, phylogenetic instability results in changes to the contents
(inclusiveness) of taxon names, whereas under Bryant’s scheme, phylogenetic instability would result in discarding certain names
and coining new ones.This might be seen as
replacing one kind of nomenclatural instability with another.
The above recommendations remain relevant even if designated phylogenetic contexts are adopted, since they will result in
application of names to clades such that
designated phylogenetic contexts are unlikely to be violated. For instance, in Fig.
1a, applying a node-based name to the clade
FGHIJ (“the least-inclusive clade containing
F and J”) requires a designated phylogenetic
context where FGHIJ form a clade, with F
and J on opposite branches of its basal dichotomy. If basal relationships within FGHIJ
are poorly corroborated (Fig. 1a), the designated phylogenetic context might be violated (Fig. 1b, c). In contrast, applying a
stem-based name to the clade FGHIJ (“the
most-inclusive clade including H but not
C”) requires a designated phylogenetic context where FGHIJ form a clade and C is
part of the sister group of that clade, an arrangement that is unlikely to be violated
(Fig. 1b). Conversely, if basal relationships
within FGHIJ are well-corroborated, but external relationships are poorly supported
(Fig. 1d), then the reverse is true. The designated phylogenetic context of a node-based
1998
725
POINTS OF VIEW
name for FGHIJ is likely to be satisŽed (Fig.
1e), while that of the stem-based name might
be violated (Fig. 1f). Similarly, applying an
apomorphy-based name to a clade wellcorroborated by other characters obviously
means thatthe designated phylogenetic context is likely to remain satisŽed.
As discussed above, instead of using a
brief stem-based deŽnition if basal relationships within the named clade are uncertain,
and a brief node-based deŽnition if external
relationships of the named clade are uncertain, one can use either type of deŽnition
but include detailed (and thus long) lists
of relevant taxa. Such deŽnitions have the
possible advantage of including designated
phylogenetic contexts in their phrasing. Under Bryant’s proposals, Zeligidae can be
attached to the clade FGHIJ in Figure 1b
by deŽning it as “the most-inclusive clade
containing H and not C,” and appending
a designated phylogenetic context such as
ABCDE(FGHIJ). However, if long deŽnitions are not problematic, the designated
phylogenetic context can be incorporated
into the deŽnition itself, which would thus
read, “the least-inclusive clade containing F,
G, H, I, and J but excluding A, B, C, D, and E.”
This deŽnition would make sense only in the
context of phylogenetic hypotheses consistent with the arrangement ABCDE(FGHIJ);
it would be nonsensical (and thus automatically invalid) in the context of alternative hypotheses. The need for mentioning numerous taxa in such deŽnitions might be alleviated by reference to standard lists (e.g., Ford
and Cannatella, 1993; Bryant, 1996). However, it is unlikely that standard lists will
always contain groupings of taxa that coincide neatly with those required by many definitions. Furthermore, this approach partly
shifts rather than addresses the problem of
long deŽnitions, as the deŽnitions would
then not stand alone but be partly contained
in the relevant lists.
Thus, if long deŽnitions are not problematic, both stem-based and node-based
deŽnitions can be phrased to include designated phylogenetic contexts. Similarly,
apomorphy-based deŽnitions can also be
elaborated to restrict their applicability to
phylogenetic hypotheses where the apo-
morphy is uniquely derived: “the clade diagnosed by the unique origin of apomorphy X.” Such deŽnitions will be invalid in
phylogenetic contexts where apomorphy X
originates multiple times. Stem-, node-, and
apomorphy-based deŽnitions that include a
designated phylogenetic context also have
the advantage that their contents are at least
partly identiŽable without reference to a
cladogram. This refutes the argument that
phylogenetic deŽnitions are undesirable because they are cannot stand alone but rather
must be interpreted in the context of extraneous information such as a reference cladogram (e.g., Dominguez and Wheeler, 1997).
A recent case study has suggested that use
of phylogenetic deŽnitions of taxon names
increases their stability and explicitness and
thus results in superior taxonomies (Cantino
et al., 1997). However, the full potential of
the phylogenetic system to improve nomenclatural stability will not be realized if only
one type of deŽnition (e.g., node-based) is
used exclusively, or if the three types of definitions are applied uncritically. Rather, in
some contexts one type of deŽnition will result in stable named clades and concise definitions, whereas in other contexts it will result in unstable named clades, cumbersome
deŽnitions, or both. In certain situations
(e.g., molecular phylogenies) some types of
deŽnition are inapplicable. Knowing when
to use each type of deŽnition will help maximize the stability, conciseness, and accessibility of phylogenetic taxonomies and will
enable systematists to communicate their
work much more clearly and effectively.
ACKNOWLEDGMENTS
I thank R. Shine and colleagues for logistical support; K. de Queiroz, D. Cannatella, H. Bryant, G. Rouse,
and two anonymous referees for helpful discussion and
comments; and the Australian Research Council for
funding.
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Received 6 November 1997; accepted 1 June 1998
Associate Editor: D. Cannatella