Tribal classification
Acacia sens. lat. is a member of tribe Acacieae within subfamily Mimosoideae of the family Leguminosae (the Pea Family). The other two significant tribes in this subfamily are the Ingeae (often considered sister to Acacieae) and Mimoseae. The Mimosoideae (which is sometimes treated as a distinct family, the Mimosaceae) contains about 50–60 genera that are distributed throughout tropical, subtropical and warm-temperate regions of the world (Elias 1981, Cowan 1998 and Lewis et al. 2005). Presently there are five tribes recognized within the Mimosoideae, namely, Acacieae, Ingeae, Mimoseae, Mimozygantheae and Parkieae (Elias 1981). Traditionally, Tribe Acacieae is regarded as containing just two genera, the very large cosmopolitan genus Acacia sens. lat. (containing more than 1 400 species) and the monotypic genus Faidherbia from Africa, West Asia and the Arabian Peninsula. (Note: Faidherbia is sometimes classified as a member of the closely related tribe Ingeae, see Polhill 1994; additional information on the classification and phylogeny of Faidherbia is given in Maslin & Stirton 1997 and Maslin et al. 2003). As summarized by Maslin et al. (2003) many uncertainties exist not only with the status of tribe Acacieae, especially in relation to tribe Ingeae and to a lesser extent tribe Mimoseae. More recently, Miller and Seigler (2015) noted that the tribal relationships within subfamily Mimosoideae are now in disarray, and that new phylogenetic data is vital for establishing an acceptable tribal and generic system of classification of the Mimosoideae.
Phylogeny & genera within Acacia sens. lat.
Over the past two decades a number of studies have examined the phylogeny and generic status of Acacia sens. lat. using cladistic analyses of morphological data (Chappill & Maslin 1995) and also molecular sequence data which has been especially informative in developing a better understanding of the group. Seven of the comparative molecular studies involving species of Acacia sens. lat. have also included members of tribes Ingeae and Mimoseae, namely, Miller & Bayer (2000 & 2001), Luckow et al. (2003), Miller et al. (2003), Bouchenak-Khelladi et al. (2010), Miller & Seigler (2012) and Kyalangalilwa et al. (2013). Miller & Seigler (2012) provide a good synoptic overview of molecular studies relating to the phylogeny and classification of Acacia sens. lat. and much of which follows is based on that publication.
All seven of the above-mentioned studies have consistently shown that Acacia sens. lat. is clearly polyphyletic and comprises at least five monophyletic groups representing the five genera listed above. Acacia sens. str., Acaciella, Mariosousa and Senegalia are nested within a paraphyletic grade which includes genera of tribe Ingeae, while Vachellia is nested within, or sister to, a clade containing the paraphyletic tribe Mimoseae.
The congruence between these molecular studies, and the congruence between clades derived from these studies and prior morphological groupings, provides powerful evidence for the polyphyly of Acacia sens. lat., mandating against it being retained as a single genus. This is widely accepted within the international legume community. The genus is now widely accepted as having been split (and some further splitting, particularly within Senegalia, will occur in the future).
Miller & Seigler (2012) demonstrate strong support (97% parsimony bootstrap) for the clade containing Acacia sens. str. etc. + Ingeae, with Vachellia excluded. Lower support values (71–83%) are reported for this clade in the other previous plastid datasets referred to above; however, Brown et al. (2008), using rDNA sequence data, obtained 97% parsimony bootstrap support for this node (although their analysis did not include species of tribe Mimoseae). Vachellia is shown by Miller & Seigler (2012) as strongly supported (97% parsimony bootstrap) within a paraphyletic tribe Mimoseae.
As already noted, all five currently recognized genera have been shown to be monophyletic (although not all analyses included all genera). These genera are congruent with five infra-generic groups previously recognized within Acacia sens. lat. based on morphological data (see Table below). Miller & Seigler (2012) and others have reported robust support values for Acacia sens. str., Acaciella, Mariosousa and Vachellia (all having >90% parsimony bootstrap values) but only moderate support for Senegalia (72% bootstrap value). It is likely that future studies will recognize further generic segregates from within Senegalia (e.g. the ‘skleroxyla’ group from South America).
Phylogeny of Acacia sens. lat. within the context of tribes Ingeae and Mimoseae. Adapted from Figure 1 in Miller & Seigler (2012) and reproduced here with the kind permission of Joe Miller.
Classification history of Acacia sens. lat.
The classification history of Acacia and nomenclatural implications associated with splitting the genus is discussed elsewhere on WorldWideWattle; a simplified version of these issues, and in particular, the nomenclatural impacts, is also given.
The following synopsis outlines the current higher-order classification of Acacia sens. lat.
The circumscription of Acacia had become relatively stable since about the mid-19th century, following a series of papers between 1842 and 1875 by the British botanist, George Bentham. The genus was first described by Philip Miller in 1754 and until 1842, when Bentham clearly defined it’s limits (by restricting the name Acacia to Mimosoid plants having numerous free stamens), a number of species which are now referable to genera within tribes Ingeae and Mimoseae, had been referred to it. In 1875 Bentham published his final (worldwide) classification of Acacia and this remained in place for about 100 years. The first major rearrangement of Bentham’s scheme was undertaken by the French botanist, Jacques Vassal in 1972. Vassal divided Acacia into three subgenera, Acacia, Aculeiferum and Heterophyllum (=Phyllodineae), and this work assisted in providing the conceptual framework for Pedley’s (1986) formal split of the genus into three genera. Initially Pedley (1978) viewed Acacia as comprising three large subgenera, as Vassal had done previously. However, in 1986 Pedley raised the rank of these groups to that of genus, Acacia, Senegalia and Racosperma respectively. Although it was generally accepted at the time that Acacia comprised a number of disparate groups, Pedley’s proposal was not widely adopted by the botanical community. Of primary concern was the widespread nomenclatural disruption that would ensue from splitting this enormous cosmopolitan genus. It was considered that more comprehensive information was needed in order to make informed decisions, and in particular, the need for broad-based comparative studies of Acacia that included genera from tribes Ingeae and Mimoseae was identified. Discussion of the reasons for not accepting Pedley's generic classification are outlined in Maslin (1987 and 1988, the latter paper was reproduced in 1989). Pedley published a defence of his scheme in 1987 and 1989.
Subsequent to 1986 there was an accumulation of much new data derived from both morphological and molecular genetic studies which has contributed to a better understanding of the classification and phylogeny of Acacia. A summary of these data, to 2003, is given in Maslin et al. (2003). The new information confirmed that the then subg. Acacia and subg. Phyllodineae were monophyletic, however, subg. Aculeiferum was not monophyletic. Maslin et al. (2003) recognize three monophyletic assemblages within this last subgenus and suggested that at least five genera can be recoginzed within Acacia sens. lat. ,. At that time the nomenclatual issue concerning the application of the name Acacia had not been resolved, therefore the genera at that time were presented under the following names:
- AcaciaA (based on Acacia [the former] subgenus Acacia; a pantropical group containing about 163 species).
- Senegalia sens. str. (based on Acacia subgenus Aculeiferum; a pantropical group containing 203 species).
- Acaciella (based on Acacia subg. Aculeiferum section Filicinae; a group of 15 species confined to the Americas).
- Genus xB (an undescribed genus based on a group of 13 species related to Acacia coulteri; confined to the Americas).
- RacospermaC (based on Acacia subgenus Phyllodineae; a group largely confined to Australia and containing 987 species).
AThis genus is now (2015) known as Vachellia. BThis genus is now (2015) known as Mariosousa. CThis genus is now (2015) known as Acacia (sens. str.)
In more recent time, as discussed above, further genetic evidence has confirmed the above five-generic classification, however, it is likely that futher splitting, particularly of Senegalia, will occur in the future.
The following table summarizes the main classification schemes that are discussed above.
Bentham (1875) |
Vassal (1972) |
Pedley (1978) |
Pedley (1986) |
Current classification (2015) |
|---|---|---|---|---|
ACACIA |
ACACIA |
ACACIA |
ACACIA |
VACHELLIA |
Ser Vulgares |
Subg Aculeiferum |
Subg Aculeiferum |
SENEGALIA |
SENEGALIA |
|
Subg Phyllodineae |
Subg Phyllodineae |
RACOSPERMA |
ACACIA |
Subser Pungentes |
|
|
|
|
Sec Lycopodiifoliae |
Sec Lycopodiifolia |
|||
Ser Pulchellae |
Sec Pulchelloidea |
Sec Pulchellae |
Sec Pulchella |
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