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DENDRON » No/Nr: 43 » November 2011
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review
review
The traditional classification of Acacia s.l. was based
mainly on the ideas of Bentham (1875) who considered
it to be a single genus, containing six series. The most
widely used recent classification is that of Vassal
(1981)—one genus, three subgenera (see Table 2).
In 1986 Pedley proposed that the three subgenera
should each be raised to generic rank, namely
Acacia, Senegalia Raf. and Racosperma Mart. (Table
2), but taxonomists have been reluctant to accept
this classification because of an apprehension of the
nomenclatural turmoil that it would entail (Orchard
& Maslin 2003). Recent taxonomic work done on
the genus and its relatives in the tribes Ingeae and
Mimoseae have greatly expanded our knowledge of
the groups, however (Maslin et al. 2003). This includes
evidence from morphology, palynology, biochemistry,
molecular and cladistic studies and led to a general
Pre- Vienna ICB names
(A. nilotica as type)
Acacia
CLASSIFICATION AND NOMENCLATURE OF THE GENUS ACACIA
Pedley’s classification
3. Role of the ICBN
It is important at this stage, before we continue to
the discussion on the modern controversies around
the name Acacia, to define clearly the terminology
used in the field of systematics. According to the
13 th Edition of Henderson’s dictionary of biology
(Lawrence 2005), systematics is the study of the
identification, taxonomy and nomenclature of
organisms, including the classification of living things
with regard to their natural relationships. Taxonomy
is the analysis of an organism’s characteristics for
the purpose of classification, whereas classification
is the arrangement of living organisms into
groups on the basis of observed similarities and
differences. Most modern classification systems
try to reflect degrees of evolutionary relatedness
(Lawrence 2005). Identification is the process of
associating an unknown taxon with a known one or
the recognition that the unknown is new to science
and the assignment of names using a formal system
belief among taxonomic workers that Acacia s.l.
should in fact be five segregate genera (Orchard &
Maslin 2005).
In 2005 Orchard & Maslin described these five
groups as follows (Table 2): “The largest of these,
with about 960 species is the current A. subg.
Phyllodineae (DC.) Ser. (= subg. Heterophyllum
Vassal), for which the name Racosperma is available.
The next largest is A. subg. Aculeiferum Vassal with
about 203 species, and for which the name Senegalia
is available. The third major group, presently known
as A. subg. Acacia, contains about 161 species,
including the current type of Acacia, A. nilotica.
The name Vachellia is available for this group. The
remaining two groups are small and compromise 15
and 13 species respectively.” These last two smaller
groups arise from within A. subg. Aculeiferum.
With retypification
(A. penninervis as type)
Post- Vienna ICB names
Without retypification
(A. nilotica as type)
subg. Acacia Acacia Vachellia Acacia
subg. Aculeiferum Senegalia
sect. Spiciflorae Senegalia Senegalia
sect. Filicinae Acaciella Acaciella
A.coulteri group Mariosousa Mariosousa
subg. Phyllodineae Racosperma Acacia Racosperma
TABLE 2.– Summery of the various classification systems used for the genus Acacia s.l.
is called nomenclature (Simpson 2006).
Scientific nomenclature allows people to
communicate about organisms and to store and
retrieve information about these organisms.
Simpson (2006) summarises the need for names
well: “Botanical names serve as symbols of a group
of natural entities for the purpose of communication
and data reference.”, while the preface to the 2005
International Code for Botanical Nomenclature
(ICBN) (available at http://ibot.sav.sk/icbn/main.
htm) states that “Unambiguous names for organisms
are essential for effective scientific communication
(and) names can only be unambiguous if there
are internationally accepted rules governing their
formation and use.” Nomenclature and classification
systems are working tools and should be as stable
as possible, because if it changes continually, it
will cease to be meaningful. Taxonomists, however,
have been criticized for changing too many things
too often (Orchard & Maslin 2005). In an attempt
to stabilize and regulate plant nomenclature, the
International Code for Botanical Nomenclature
(ICBN) has been developed over decades in order
to try to increase stability of names. Several articles
of the code have been designed and incorporated
specifically to help facilitate the conservation of
names in order to minimise name changes (see Box
1 for examples of conserved names in use today).
Conserved Name Reason for Conservation Key Reference
Hedysarum
(Leguminosae: Papilionoideae)
Leucaena
(Leguminosae: Papilionoideae)
Centaurea
(Asteraceae)
Bossiea
(Leguminosae)
Research showed that the name
Hedysarum would be restricted to
H. subg. Hedysarum, containing
only six species. A new name will
then be required for the remaining
members of Hedysarum s.l.—
approximately 100 species.
Leucaena was conserved to a new
type when it was discovered that
the basionym of the previously
accepted type belonged to
Acacia. This would have caused
Leuceana to become a synonym
of Acacia.
The accepted type of Centaurea
was discovered to belong to a
small group of about 32 species
earmarked for segregation to
Bielzia. The remaining 400–700
species thus needed new names.
Bossiae (59 spp.) and Platylobium
(4 spp.) are to be merged, but
Platylobium has priority of
publication. The conservation of
Bossiae would thus be better for
nomenclatural stability.
BOX 1.– Examples of other successfully retypified and conserved names.
4. Evidence for splitting the genus
It has long been known, or at least strongly suspected,
by many taxonomic workers that the genus Acacia s.l.
is polyphyletic (Pedley 1986; Orchard & Maslin 2003).
This means that the genus is actually an artificial
construct consisting out of several fairly unrelated
genera. While the majority of Acacia s.l. species are
characterized by numerous free filaments, there
are no synapomorphic characters that support their
positioning as a natural group (Kergoat et al. 2006). The
current view is that the group should be divided into
five separate genera in order to satisfy the requirement
of monophyly. In the following subsections, evidence is
provided for the five genera point of view.
4.1 Established evidence
The idea that Acacia s.l. is not a homogeneous
group is not new. A 1979 review by Ross outlined
CLASSIFICATION AND NOMENCLATURE OF THE GENUS ACACIA
The ICBN regulates primarily two basic activities,
namely (1) the naming of new, undescribed or
unnamed taxa and (2) correctly renaming previously
named taxa which have been divided, united,
transferred or changed in rank. It is this second
activity that we are interested in in this case, as the
genus Acacia has been changed in rank.
Choi & Ohashi (1998)
Hughes (1997)
Greuter et al. (2001)
Ross (2004)
review
review
many differences between the two mainly African
subgenera of Acacia s.l., based on morphology,
palynology, genetics, anatomy and biochemistry
(Table 3). It is expected that there would be an
even number, if not greater number, of differences
between the African and Australian subgenera
based on these criteria, but literature on this could
not be accessed.
It is easy to see how these groups could have been
lumped together for so long. Consider for example
the thorns: One can easily mistake the presence
of thorns in both groups as a common character,
but on closer inspection it turns out that they are
not homologous. In Vachellia the thorns are in
fact spines derived from the stipules, where as in
Senegalia, they are prickles derived from epidermal
outgrowths. These two seemingly similar characters
thus represent a very different evolutionary history.
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DENDRON » No/Nr: 43 » November 2011