A review of the classification of Acacia (Leguminosae, Mimosoideae)

This paper reviews the history of the systematics of Acacia s.l. , providing a comparison of recent phylogenies to previous classifications, with a focus on Acacia s.s. Herein, Acacia s.s. is used in the sense of the segregated genus based on A. penninervis, and excluding Senegalia, Vachellia, Acaciella, and Mariosousa. Problems have been apparent with the classification of Acacia s.l. for many years and there is now general recognition that Acacia s.l. is polyphyletic. Acacia s.s. , a largely Australian group with close to 1000 species, has been shown to be monophyletic. Although molecular phylogenetic studies in recent years have clarified the relationships of some groups within Acacia s.s. , this diverse clade lacks a phylogenetic classification. Recent evidence has provided renewed support for the recognition of a redefined section Pulchelloidea sensu Vassal, and although formal taxonomic changes at this time are regarded as premature, it has been demonstrated that taxonomic groups in Acacia s.s. based upon one or two “key” characters are not monophyletic. Muelleria 26(1): 10-26 (2008) Introduction The genus Acacia Miller, if treated in the broad sense, includes c.1450 species (Lewis et al. 2005, p. 3) with species in Africa, the Americas, Asia and Australia, and forms the second most speciose genus in the family Leguminosae after Astragalus (Maslin 1988; Mabberley 1997). If treated in the strict sense following the re-typification of Acacia with an Australian type (Orchard & Maslin 2003; Maslin 2008), Acacia s.s. (formerly Acacia subgenus Phyllodineae, synonym Racosperma), includes 987 species (see Fig. 1 for distribution) with 975 species in Australia (Maslin 2004), and is still the second largest legume genus by a considerable margin (Lewis et al. 2005). Acacia s.s. comprises the largest genus of plants in Australia (Maslin 1995). No species currently occur in New Zealand, although fossil evidence suggests that the genus was once present there (Guinet 1981; Martin 1994). Several species of Acacia s.l. are common in the Middle East, and references to members of the genus date from early in recorded history. For example, the ancient Egyptians had a hieroglyphic symbol for Acacia, and the Bible describes the Ark of the Tabernacle as being made of Acacia wood (Moldenke & Moldenke 1952). It is not surprising that a group of plants known from such early times has such a long and complex systematic history. Herein the segregated generic names for Acacia s.l. will be used following the re-typification of Acacia with an Australian type, namely: Acacia s.s. (formerly Acacia subgenus Phyllodineae, synonym Racosperma), Vachellia (formerly Acacia subgenus Acacia), Senegalia (formerly Acacia subgenus Aculeiferum s.s.), Acaciella (formerly Acacia subgenus Aculeiferum section Filicinae), and Mariosousa (formerly Acacia subgenus Aculeiferum “Acacia coulteri group”). Further explanation of these names and their application can be found in Maslin (2008). However, when comparing alternative classifications and results, the original names used by each author are retained (where possible), to simplify the summary of the original literature. In recent times the need for a phylogenetically based classification of Acacia s.l. , to guide workers in a variety of fields, has become increasingly apparent. Examples include studies of host-parasite co-evolution (Crespi et al. 2004); plant physiology (Pohlman 2005; Warwick & Thukten 2006);