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DENDRON » No/Nr: 43 » November 2011
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CLASSIFICATION AND NOMENCLATURE OF THE GENUS ACACIA
clear that Acacia s.l. is a very heterogeneous group
and that it would make sense to divide it into five
more homogeneous genera, because it would greatly
improve the stability and predictive capabilities of
the classification system. The split would however
negatively affect our ability to access information on
the taxa that now use new names, because a name
acts as a key to accessing information. But over time
and with the correct use of synonyms, this problem
will become less pronounced. Moving the type
specimen from the African A. nilotica to the Australian
A. penninervis, however, was a much more contentious
and hotly debated proposal. The argument for and
against this move is discussed in the next section.
Acacia s.s
Ingeae
Acaciella
Mariosousa
Senegalia
Mimoseae
Vachellia
Mimoseae
Caesalpinieae
FIGURE 2.– Simplified phylogenetic tree of the
five genera within Acacia s.l. and tribes Ingeae,
Mimoseae and Caesalpinieae. Adapted from
Bouchenak-Khelladi et al. (2010).
5. Retypification
Splitting the genus Acacia into five segregate genera
means that only one of the five newly delimited
genera can keep the generic name of Acacia.
Post-Vienna ICB names
(A. penninervis as type)
According to the ICBN the group that contains
the type specimen of the name gets to keep that
particular name. In the case of Acacia, the generic
type was A. nilotica, a member of A. subg. Acacia,
containing ca 160 taxa. This meant that once the
genus was split, only some 12% of the more than
1400 species of Acacia s.l. will still be able to use
the name (see Table 4). Orchard and Maslin (2003)
proposed to move the generic type specimen of
genus Acacia to a member of the largest splinter
group, A. penninervis of the subg. Phyllodineae. This
would ensure that about 72% of the original Acacia
s.l. species can keep using the name Acacia. When
just considering pure numbers, retypification looks
like a brilliant idea and it is difficult to think how
anyone can disagree with it, but the retypification of
Acacia was probably one of the most hotly debated
issues in modern plant taxonomy. See also Box 1
for other examples of retypification and Table 1 for
dates of retypification.
As a result of the generic split, about half of Africa’s
thorn trees will become Senegalia. The contest for
use of the generic name Acacia is thus restricted
to A. subgenera Acacia and Phyllodineae. Because
A. subg. Phyllodineaea has a near exclusive
Australian distribution and A. subg. Acacia is
strongly associated with especially Africa, but also
South America, this became an ‘Australia versus
the rest of the world’ debate. Most of the arguments
for and against retypification are summarised in
Table 5 while some of the more important ones are
discussed in more detail below.
Retypification is a well tried and accepted procedure
allowed by the ICBN and the precedent exists that
the name of a large genus should be conserved to
advance nomenclatural stability in such cases (see
Box 1 for examples). Luckow et al. (2005) however
argue that each case should be examined carefully, in
particular the impact on the taxon that stands to lose
Sp. numbers and distribution
Americas Africa Asia Australia Total
Vachellia 52 83 32 9 163
Senegalia 79 74 48 2 194
Acaciella 15 0 0 0 15
Mariosousa 13 0 0 0 13
Acacia 0 1 12 1017 1021
Total 161 158 92 1028 1406
TABLE 4.– Species numbers of Acacia s.l. members corresponding to geographic areas of major
occurrence. Adapted from Thiele et al. (2011).
CLASSIFICATION AND NOMENCLATURE OF THE GENUS ACACIA
PAPER-BARK THORN, Acacia sieberiana subsp. woodii – LESHIBA WILDERNESS » Photo: Naas Grové
the use of the original type. They quote from a paper
by McNeill et al. (2003) on the conservation of names:
“Committees will not be sympathetic to proposals to
avoid disadvantageous change in usage in one part of
the world at the expense of creating disadvantageous
change in another. These situations are what the
principles of priority are for”. Basically, Luckow et
al. (2005) said that with such a controversial, evenly
contested and emotionally loaded question, such as
that of Acacia, the principle of priority should prevail.
The argument for or against retypification should not be
based purely on numbers, but the share of the world’s
population that would be affected by such changes
should also be considered. Although Acacia s.s. is very
speciose and contains about 72% of Acacia s.l. taxa,
the majority of these species are narrow endemics,
while Vachellia has a much wider distribution (Luckow
et al. 2005). Thus the change to Vachellia will affect
more people in more countries around the world than
the change to Racosperma will, even if there are about
six times more taxa in Racosperma than Vachellia.
Orchard & Maslin (2005) are of the opinion that gross
numbers are very important, irrespective of the
distribution of individual taxa. They argue that while
widespread species may impact on more floras, rare
species will feature much more widely in conservation
literature and legislation.
Another important focal point in the argument for
or against retypification was that of economic and
ecological importance of the two groups in question.
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Many acacias of economic importance are of the subg.
Phyllodineae and are grown in more than 70 countries
where they are estimated to cover over 2 million
hectares in commercial plantations (Orchard & Maslin
2003). There are thus many countries, industries
and activities that are affected by the change to
Racosperma. Weed management, floriculture, forestry
(timber, pulp, tannins and fuel wood) agriculture and
land rehabilitation to name a few. In terms of ecological
importance, it is very difficult to compare two different
groups from two different continents with one another.
There are many varied views in the literature as
to which group is more important to its respective
continent, but I am of the opinion that we cannot make
such a call based on current knowledge.
Lastly, an often over looked fact is that the use of
Racosperma would be unfortunate as the gender will
change from feminine (Acacia) to neuter (Racosperma)
and this will affect the specific epithets. For example:
Acacia adunca would become Racosperma aduncum
and users that are not familiar with Latin (which I am
pretty sure includes most users) will be tempted to
‘correct’ it to Racosperma adunca (Orchard & Maslin
2003). The use of Vachellia and Senegalia necessitate
no gender changes as both are feminine.
The retypification issue of Acacia was a very evenly
contested debate and both side presented very
convincing arguments, but the battle has been fought
and a conclusion has been reached. Whether it was
the best route to follow or not, only time will tell.
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DENDRON » No/Nr: 43 » November 2011