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DENDRON » No/Nr: 43 » November 2011
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CLASSIFICATION AND NOMENCLATURE OF THE GENUS ACACIA
Classification and nomenclature
of the genus Acacia (Leguminosae), with emphasis on Africa
Abstract
The widespread and well-known genus Acacia
has experienced a huge amount of taxonomic
turmoil over the past couple of decades. In this
contribution the nomenclatural history of Acacia is
outlined. Evidence for the recent split of the genus
into five segregate genera is provided and the
retypification from the African species A. nilotica to
the Australian A. penninervis is discussed critically.
The International Code for Botanical Nomenclature
(ICBN) is introduced and discussed as a set of
guidelines for taxonomic change. Lastly, some ideas
are provided for plant name users of how to deal
with this nomenclatural inconvenience.
“No other taxonomic proposal
has ever had the potential for
disruption to nomenclature on a
continental scale as the one to split
Acacia.” – Orchard & Maslin 2005
1. Introduction
Convincing evidence has been accumulating
over the past two to three decades that the large,
well-known and charismatic genus Acacia Mill. is
polyphyletic (Maslin et al. 2003; Orchard & Maslin
2003). Polyphyletic refers to a group derived from
different common ancestors. The ideal in plant
classifcation is to recognize monophyletic groups.
In the case of a monophyletic group all members
are derived from the same recent common ancestor.
Many taxonomic workers now agree that Acacia
senso lato (as traditionally defined), meaning Acacia
in a broad sense, should in fact be five segregate
genera, because five separate monophyletic groups
can be recognised within the genus (Maslin et al.
2003). In 2005 the genus was split and retypified
in accordance to Proposal 1584 by Orchard &
Maslin (2003). This caused a plant nomenclatural
uproar, the likes of which had never been seen and
hopefully will never be seen again. The aim of this
contribution is to review the developments in Acacia
s.l. phylogeny and its impacts on the classification
and the nomenclature of the group, with emphasis
on Africa. I will further attempt to critically evaluate
the split and retypification of the genus and provide
William a. Haddad
some thoughts on how to cope with this great
nomenclatural inconvenience.
2. Background on the genus Acacia
Although the palaeobotanical evidence is
inconclusive, it is generally believed that the genus
Acacia s.l. evolved in the tropical lowland forests
of West Gondwana, but there is no agreement as
to which section should be considered ancestral.
The various sections in existence today must
have evolved from the original form in secondary
centres where the climatic or edaphic factors
were different (Ross 1979). The oldest Acacia s.l.
fossil was found in Tanzania and dated at 46 mya
(Bouchenak-Khelladi et al. 2010).
The highly complex nomenclatural history
(outlined in Table 1) of the genus Acacia s.l.
started with its formal description by Phillip
Miller in 1754. It was not until 1842, however, that
the generic limits were clearly defined by George
Bentham by restricting the name to mimosoid
plants with numerous free stamens (Maslin et
al. 2003). The genus was only typified in 1913
by Britton and Brown to the African species A.
scorpioides (L.) W.Wight (=A. nilotica (L.) Delile).
Since that time many authors have proposed
different ways to subdivide the genus, making use
of various different characters (Bentham 1875;
Ross 1979; Vassal 1981; Pedley 1986; Orchard &
Maslin 2003).
Acacia, in the broad sense, is the second largest
genus in the family Leguminosae (Fabaceae s.l.)
or the largest genus in the family Mimosaceae,
if considered under an alternate classification
system, with about 1540 living taxa (specific
& intra-specific)(Orchard & Maslin 2005). It is
widely distributed across the world’s tropics and
sub-tropical zones with centres of diversity in
Australia (± 1000 species), the Americas (±180
species), Africa (± 160 species), Asia (±100
species) and a couple of species on the Pacific
Islands (Thiele et al. 2011) (Figure 1). As of yet,
there are no explanations for this remarkably
disjunct geographic distribution (Bouchenak-
Khelladi et al. 2010). Ross (1979) describes the
ecological role of Acacia s.l. very satisfactorily:
“There is probably no group of trees or shrubs
in Africa that can rival Acacia in the combined
importance of its ecology and extent of its
geographical range. Almost everywhere from
South Africa northwards to the Mediterranean,
where the climate is reasonably dry, there will
CLASSIFICATION AND NOMENCLATURE OF THE GENUS ACACIA
Year Nomenclatural Event Key Reference
1754 Genus Acacia described. Miller (1754)
1842 Generic limits defined
1875 Considered one genus with six series. Bentham (1875)
1913 Genus Acacia typified to A. nilotica.
1981 Considered as one genus with three subgenera. Vassal (1981)
1986 Proposal to raise subgenera to three individual genera. Pedley (1986)
2003
Proposal 1584 to split genus into five genera and to move the type of
name Acacia to A. penninervis.
Orchard & Maslin
(2003)
2004 The IAPT Committee for Spermatophyta recommends proposal 1584. Brummitt (2004)
2005
2011
Proposal 1584 is controversially accepted at the nomenclature
section of the 16th IBC in Vienna. The type is moved and five different
genera are now recognized.
An attempt is made at the 17th IBC in Melbourne to overturn the
decisions made in Vienna. The attempt failed and the type remains an
Australian species.
TABLE 1.– Simplified nomenclatural history of the genus Acacia.
180
160
Acacia be found, usually frequently and often in
dominant abundance. Only in the evergreen forest
regions of Africa do Acacia play a comparatively
subordinate role”.
100
1000
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FIGURE 1.– Distribution map showing the range of Acacia s.l. with approximated numbers of endemic taxa
per continent. Image from www.worldwidewattle.com and data from Thiele et al. (2011).
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DENDRON » No/Nr: 43 » November 2011