NOTE: The article below titled ‘Proposed name changes in Acacia’ summarizes, in a simplified way, the main issues involved with division of Acacia (a more detailed discussion of these issues is also presented on WorldWideWattle). This article was originally published by the Association of Societies for Growing Australian Plants (ASGAP) in their journal Australian Plants (Maslin 2003) and on the web at Australian Plants on Line. An slightly modified version of the paper will also be published by in the Newsletter of the Western Australian Wildflower Society, Perth, W.A., and the Newsletter of the Eurobodalla Regional Botanic Gardens, Batemans Bay, N.S.W. The article below is an updated of this slightly modified version.
Based primarily on the results of recent molecular studies there is a growing consensus among specialists that a formal division of Acacia into a number of segregate genera is likely to take place in the near future. When this happens it will trigger the need for several new generic names and as a result there will be considerable nomenclatural disruption worldwide, at least in the short term. One of the important questions relating to this issue is: to what group should the name Acacia be applied?
The possibility that most Australian Acacias may require a new generic name when the genus is dismembered is of concern to various sections of the Australian community (see articles by Burns 2003, Simmons 2003 and Williams 2003). Acacia is an iconic Australian group which, apart from being the largest genus of vascular plants in the country, has both significant historical and cultural significance to Australians. At a global scale the ‘Australian group’ is largest in terms of species numbers, it is also very important economically. For these reasons a formal proposal by Orchard & Maslin (2003) has been published which aims to minimise the nomenclatural inconvenience associated with splitting the genus by advocating the retypification of Acacia with a member of ‘Australian group’ of the genus. This proposal will now be considered by the international Committee for Spermatophyta and it is hoped that a decision will be forthcoming in the not too distant future.
Note: The species numbers cited in the article below have now been updated.
Proposed name changes in Acacia
By B.R. Maslin (Dept of Conservation and Land Management, Perth, W.A.)
Two articles on the impending split of Acacia and the generic name changes that will subsequently occur recently appeared in the public arena, one in Nature Australia (Low 2002) and the other in the Sydney Morning Herald (Woodford 2002). The article by James Woodford was balanced and well written, however, because of space constraints he was not able to deal with the issues in the detail that I would have liked. I am therefore taking this opportunity to provide these details and also to provide additional information that may help clarify matters concerning this important issue.
As will be discussed below there is a growing consensus among specialists that a formal division of Acacia into a number of segregate genera is necessary and is likely to take place in the near future. When this happens it will trigger the need for several new generic names and one of the very important issues is what group should the name Acacia be applied to. If these names are applied in accordance with their presently designated types then the name Acacia would be applied to a relatively small cosmopolitan group of Acacias which is very poorly represented in Australia. It would then follow that the majority of the 955 species of Acacia that occur in this country would require another generic name, and it has been proposed that the name Racosperma be used. Such a change involving this large and important iconic Australian genus (which is our official floral emblem) has understandably caused concern to a number of people. The International Code of Botanical Nomenclature (Greuter et al. 2000), however, does make provisions for minimizing the impact of such changes in certain cases, and it is these provisions that Tony Orchard (Australian Biological Resources Study, Canberra), Judy West (Centre for Plant Biodiversity Research, Canberra) and I are attempting to invoke. Basically we are proposing that, in the interest of nomenclatural stability, the genus Acacia be retypified (see discussion below). One of the consequences of this action, if successful, would be that the names of most Australian species of Acacia will not change. However, as discussed below, this is not our only motivation for taking this action.
Acacia is the second largest genus in the family Leguminosae, with about 1350 species currently recognized. These species are distributed throughout tropical and warm temperate areas of the world, with the largest concentrations occurring in Australia (955 species), but there are also high numbers in the Americas (about 185 species), Africa (144 species) and Asia (89 species).
The current, commonly used infrageneric classification of Acacia recognizes three groups, namely:
- Subgenus Acacia: about 160 species which are widely distributed in Africa (73 species), the Americas (about 60 species), Asia (36 species) and Australia (9 species). The Australian species are mostly confined to the tropical north of the continent, only A. farnesiana extends southwards through more arid areas but it is not likely that this species is a true native of Australia.
- Subgenus Aculeiferum: 231 species which are also widely distributed in the Americas (125 species), Africa (69 species), Asia (43 species) and Australia (2 species). The Australian species are confined to northern Queensland.
- Subgenus Phyllodineae: 960 species which are largely confined to Australia (less than 20 species occur outside the continent where they extend eastwards to some islands of the Pacific, north to the Philippines and west to Madagascar).
Therefore, by far the largest numbers of species occur in subgenus Phyllodineae and these are mostly confined to Australia. The other two subgenera are uncommon in Australia; they have cosmopolitan, pan-tropical distributions where they occur roughly in equal numbers, furthermore, in some areas of Africa and the Americas species of these two subgenera can be found growing together.
Acacia was originally described by Philip Miller in 1754. Miller’s concept of the genus was very broad and it included 24 species that occurred in Africa and America (many of these species are now shown to belong to genera other than Acacia). What is particularly relevant to the present discussion is that one of the species included in Miller’s paper was subsequently selected as the type species for the genus Acacia. This species is A. scorpioides (which is now known as A. nilotica, a species that occurs in Africa and Asia). This means that under normal circumstances the name Acacia should be applied to the group in which A. nilotica occurs. The significance of this will become apparent from the discussions below. Subsequent to Miller’s publication there were almost 30 new genera described for various segregates of Acacia, but only one of these genera, Faidherbia (which contains a single species that grows in Africa and western Asia) is currently accepted. The genus Racosperma which is discussed below is also one of the 30 genera that were subsequently described.
Acacia nilotica in Saudi Arabia. Photo by Steve Midgley.
The first significant re-arrangement of the long-established classification of Acacia that had been laid down by George Bentham in the mid-nineteenth century was done by Jacques Vassal in 1972. Vassal’s new classification recognized the three subgenera listed above. In arriving at his scheme Vassal took into account information derived from his study of seeds, seedlings and stipules and also the pollen work done by his colleague Phillipe Guinet. In 1986 Les Pedley from the Queensland Herbarium in Brisbane re-assessed the classification of Acacia in the light of Vassal’s work and other data, and proposed that three genera (corresponding to Vassal’s three subgenera) be recognized, namely:
- Acacia (= subgenus Acacia)
- Senegalia (= subgenus Aculeiferum)
- Racosperma (= subgenus Phyllodineae)
While many botanists agreed that Acacia included a number of more or less disparate groups, it was considered imprudent, in 1986, to accept a generic status for these as proposed by Pedley and therefore this classification was not widely adopted by either the botanical or non-botanical community. Of primary concern was the widespread nomenclatural disruption that would ensue from splitting this enormous cosmopolitan genus. On taxonomic grounds it was considered that the evidence presented by Pedley to substantiate his recognition of three genera was inconclusive and/or incomplete. It was generally considered that there was a need to acquire additional critical information upon which informed decisions could be made concerning the generic status of Acacia. Also, there was a recognized need for broad-based comparative studies of Acacia that would include genera from tribes such as the Ingeae and Mimoseae. These issues are discussed in Maslin (1989; see also Pedley 1987 & 1989 where he defends his scheme).
Subsequent to 1986 a considerable amount of work has been undertaken to re-assess the generic status of Acacia by examining in detail the classification and phylogeny (evolution) of the group. Of particular relevance are the cladistic analyses of Chappill (1995) and Chappill & Maslin (1995), and the exciting new molecular information that is being generated by Joe Miller and his colleagues (e.g. Miller & Bayer 2001, Luckow et al. in press). These were broad-based studies that included not only species of Acacia and Faidherbia but also genera from related taxonomic groups. Although this genetic work is still on-going, the current results clearly show that Acacia as currently defined must be divided into a number of genera. What is not clear, however, is how many genera should be recognized. Although these new studies lend general support to the Vassal/Pedley classification of Acacia into three groups, the most recent evidence suggests that the dismemberment should go even further, and that at least five genera should be recognised (see Maslin et al. 2003), namely:
- Acacia (= subgenus Acacia): c. 163 species [Africa - 73; Americas – c. 60; Asia - 36; Australia - 9.]
- Senegalia (= subgenus Aculeiferum sens. str.): 203 species [Africa - 69; Americas – c. 97; Asia - 43; Australia - 2.]
- Racosperma (= subgenus Phyllodineae): 961 species [Madagascar & Mascarenes: 2; Asia - 10; Australia & Pacific- 956.]
- Acaciella (= subgenus Aculeiferum section Filicinae): 15 species [Americas.]
- Genus x (= an undescribed genus comprising species of the ‘Acacia coulteri group’ in subgenus Aculeiferum): 13 species [Americas.]
Relevant classification schemes from Bentham (1875) to Maslin et al. (2003) are summarized in Table 1.
Although there is consensus among specialists that the formal division of Acacia will take place in the near future a major impediment remains: when the genus Acacia is divided, and the provisions of the International Code of Botanical Nomenclature are followed, then very large numbers of new combinations (names) will be needed at the species and infraspecific level. Under the scenario whereby names are applied in accordance with their currently designated types (i.e. with A. nilotica treated as the type species of the genus) it would mean that the name Acacia would be applied to one of the smallest groups, namely, subgenus Acacia. This subgenus contains about 163 species, representing only 12% of the genus as presently defined. The largest group, subgenus Phyllodineae, would need a new name (Pedley has proposed that it be called Racosperma); this group contains 961 species (representing 71%). Most of the species currently ascribed to the third subgenus, Aculeiferum, would become known as Senegalia (although as seen from Table 1 this subgenus is likely to be split into three genera, Senegalia – 203 species, Acaciella – 15 species and genus ‘x’ – 13 species: these combined species represent 17% of the genus as currently defined). These proposed name changes therefore involve almost 1 200 species (representing 87% of the genus as presently defined) and it would mean that most of the 967 species which occur in Australian would require a new generic name. This is the option favoured by Pedley.
There is, however, an alternative possibility, one that would minimize the disadvantageous nomenclatural impacts associated with dismemberment of the genus. It is a formal process, sanctioned by the International Code of Botanical Nomenclature, and involves conserving the name Acacia with by selecting a new type species to replace A. nilotica. Arguments have to be formally published and then judged by an international group of specialists (i.e. the Committee for Spermatophyta) who will decide on the merits of our case. A case has now been prepared and is published in the journal Taxon (see Orchard & Maslin 2003 which is available on the web). It is hoped that this matter will be resolved in the not too distant future. Essentially our argument is that it makes considerable sense to retypify Acacia with a type from the largest (i.e. subgenus Phyllodineae) because this would result in the fewest name changes worldwide. This approach is entirely consistent with the intent of Article 14 of the International Code of Botanical Nomenclature which aims to provide the means by which the interests of nomenclatural stability may be best served. If our proposal is accepted by the Committee for Spermatophyta the results would be:
- Fewer name changes would be required worldwide: only about 160 species in subgenus Acacia would need to be changed to Vachellia, compared with 960 species of subgenus Phyllodineae that would need to be changed (perhaps) to Racosperma.
- The name Acacia would be retained for the largest group, namely, subgenus Phyllodineae. Although species of this group predominate in Australia they occur worldwide as weeds and as plants of commerce; they are also used in a variety of social and environmental programs (McDonald et al. 2001).
- Least disturbance would be caused to the large international trade and industries that are built upon Australian Acacia species. A majority of the species of commerce are contained in this group; they are grown in over 70 countries worldwide, where they cover around two million hectares in commercial plantations (see Midgley & Turnbull 2003). Also, within Australia Acacias are part of the nursery trade and many species are currently being assessed for wide-scale commercial planting in the southern agricultural regions as new woody crop plants aimed at salinity control (see Maslin & McDonald, in press)
- In areas of Africa, Asia and the Americas where species of subgenus Acacia and subgenus Aculeiferum co-occur in large numbers, it will be less confusing (or certainly no more confusing) if all taxa change their names simultaneously, rather than just half. That is, Acacia as a formal generic name would disappear from these areas (species of subgenus Acacia would become Vachellia, and most species of subgenus Aculeiferum would become Senegalia). The common name Acacias could obviously continue to be used informally for the whole complex. It is worth noting here that in some areas of the world species of subgenus Acacia and subgenus Aculeiferum (Senegalia) grow side by side (i.e. they are sympatric) and if the current classification is retained it might be expected that in such cases some confusion would reign for a substantial time, while users determine which Acacias are ‘real’ Acacia and which are Senegalia. In Africa this sympatry is illustrated, for example, in Acocks (1988: 46, fig. 30) where A. erioloba and A. hebeclada (both Acacia) are photographed with A. mellifera (Senegalia) as components of Kalahari Thornveld vegetation. It is important to remember here that regardless of whether or not our proposal to retypify Acacia is accepted, more than half the species that occur naturally outside Australia will have to change their generic name (most will become known as a Senegalia).
Racosperma is the name that Pedley proposes be used for most Australian species following dismemberment of Acacia. This of course would only become an issue if the Committee rejects our argument to retypify the genus. Nevertheless, if the name Racosperma were to be adopted for the Australian species there is an issue relating to the gender of this name that will lead to confusion and nomenclatural instability at and below the species level. The name Racosperma has a neuter gender so if this name is adopted then the endings of the names of most current species, subspecies and varieties will have to change. For example, Acacia pycnantha would become Racosperma pycnanthum, Acacia axillaris would become Racosperma axillare, and so on. This will add substantially to the costs of herbaria and other institutions that have to update records because it will not be a simple ‘global replace’ of Acacia to Racosperma, each taxon will have to be considered individually. Furthermore, a potential source of confusion will be introduced, through those (non-classically aware) who interpret Racosperma as feminine and try to match the termination accordingly. Thus Racosperma angustum would be the correct form of the name of the species, but inevitably many will “correct” this to Racosperma angusta. This would be repeated across nearly 1000 new combinations.
Conclusion
Name changes will occur as a result of the fragmentation of Acacia, and in such a large and ecologically dominant genus these changes will have significant worldwide impact. Regardless of what decision the Committee for Spermatophyta makes regarding our proposal to retypify Acacia there will be those who will be disadvantaged by the coming name changes, and this is certainly unfortunate, but is unavoidable. The purpose of our proposal, however, is to minimize deleterious effects worldwide.
If most Australian Acacias have to change their generic name (perhaps to Racosperma) it will impact very considerably on this country, apart from the nomenclatural disruption and financial costs that have already been alluded to. Acacia is by far the largest vascular plant group in Australia, representing about 18.5% of the whole flora. These species are ubiquitous within the continent and are especially conspicuous in arid and semi-arid regions where they often dominate the landscape. Acacia features in virtually all floristic and ecological papers written over the past two centuries and there is a huge literature on the genus. Although a new generic name will not make this existing information inaccessible or useless, it will require that some publications at least will need to be revised. The best examples of this are the recently published Flora of Australia volumes and the associated interactive electronic key, WATTLE. These are major works, expected to be the key references to the majority of the species in the genus/genera for years to come. Finally, Acacia features strongly in folklore and traditions within Australia, and one species (A. pycnantha) is legislated as the national flower (green and gold, based on the predominant colours of most Acacias, are the national colours).
References
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