Acacia, commonly known as the wattles or acacias, is a large genus of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa and Australasia, but it has now been limited to contain only the Australasian species. The genus name is New Latin, borrowed from the Greek ἀκακία (akakia), a term used by Dioscorides for a preparation extracted from the leaves and fruit pods of Vachellia nilotica, the original type of the genus. In his Pinax (1623), Gaspard Bauhin mentioned the Greek ἀκακία from Dioscorides as the origin of the Latin name.
In the early 2000s it had become evident that the genus as it stood was not monophyletic and that several divergent lineages needed to be placed in separate genera. It turned out that one lineage comprising over 900 species mainly native to Australia, New Guinea, and Indonesia was not closely related to the much smaller group of African lineage that contained A. nilotica—the type species. This meant that the Australasian lineage (by far the most prolific in number of species) would need to be renamed. Botanist Leslie Pedley named this group Racosperma, which received little acclaim in the botanical community. Australian botanists proposed a less disruptive solution setting a different type species for Acacia (A. penninervis) and allowing this largest number of species to remain in Acacia, resulting in the two Pan-Tropical lineages being renamed Vachellia and Senegalia, and the two endemic American lineages renamed Acaciella and Mariosousa. Although many botanists still disagreed that this was necessary, this solution was eventually officially adopted at the Melbourne International Botanical Congress in 2011.
Acacia remains a widely used common name across genera.
A number of species have been introduced to various parts of the world, and two million hectares of commercial plantations have been established. The heterogeneous group varies considerably in habit, from mat-like subshrubs to canopy trees in a forest.
The genus was first validly named in 1754 by Philip Miller. In 1913 Nathaniel Lord Britton and Addison Brown selected Mimosa scorpioidesL. (≡ Acacia scorpioides (L.) W.Wight = Acacia nilotica (L.) Delille), a species from Africa, as the lectotype of the name. The genus as recognized in 1986 contained 1352 species. That year however, Pedley published a paper in which he questioned the monophyletic nature of the genus, and proposed a split into three genera: Acaciasensu stricto (161 species), Senegalia (231 species) and Racosperma (960 species), the last name first proposed in 1829 by Carl Friedrich Philipp von Martius as the name of a section in Acacia, but raised to generic rank in 1835. In 2003, Pedley published a paper with 834 new combinations in Racosperma for species, most of which were formerly placed in Acacia. All but 10 of these species are native to Australasia, where it constitutes the largest plant genus.
In 2003, Anthony Orchard and Bruce Maslin filed a proposal to conserve the name Acacia with a different type in order to retain the Australasian group of species in the genus Acacia. Following a controversial decision to choose a new type for Acacia in 2005, the Australian component of Acacia s.l. now retains the name Acacia. At the 2011 International Botanical Congress held in Melbourne, the decision to use the name Acacia, rather than the proposed Racosperma for this genus, was upheld. Other Acacia s.l. taxa continue to be called Acacia by those who choose to consider the entire group as one genus.
Australian species of the genus Paraserianthess.l. are deemed its closest relatives, particularly P. lophantha. The nearest relatives of Acacia and Paraserianthes s.l. in turn include the Australian and South East Asian genera Archidendron, Archidendropsis, Pararchidendron and Wallaceodendron, all of the tribe Ingeae.