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The classification of the genus Eucalyptus

As the eucalypts radiated across Australia from their unknown place of origin, they had to change morphologically and physiologically to adapt to the myriad of different environments. This did not result in a completely heterogeneous mass of species. As one form became a successful colonizer of a certain habitat, over time it spawned daughter species that were close morphologically but changed to a degree, either through necessary adaptation or alternatively through fortuitous events that had no effect on their survival success. Many species have no doubt become extinct in the history of the world and we can only speculate on the evolutionary history using the evidence provided by the survivors.

With this evidence, we now see the vast majority of species as being aggregated into groups of related species, all the groups being closely or remotely related to each other, assuming that Eucalyptus began as a more or less single evolutionary event and that all extant species are descendants. This is an area of Eucalyptus study that has adherents and opponents.

Several groups of species have diverged so widely from their origins they have become completely incompatible with any other. Some are single species regarded as 'dead ends', to this stage, in the evolutionary tree and are isolated in series or sections within the genus. Other groups have resulted in an enormous proliferation of species.

Primitive and/or not greatly modified species

It would be naïve to think that there is a linear progression from the most primitive to the most derived species. However, it is interesting to speculate on the concept of the most primitive surviving species in the genus and the most modified and derived. It is generally recognised that the bloodwoods are the most primitive of the large groups, as they have many of the characteristics that are believed to be those of the rainforest precursors of the genus. A unique single candidate for the most primitive is Eucalyptus curtisii of south-eastern Queensland, with its opposite dorsiventral leaves, terminal inflorescences, distinct sepals, and strange needle-like seeds.

Derived and/or greatly modified species

For the most modified species we probably should look to species in environments vastly different to that experienced by the presumed precursors. Candidates could be the snow gums which have had to adapt to the harshest of the alpine regions, although they also occupy sites of mild climate elsewhere; or Mountain Ash, Eucalyptus regnans, the tallest flowering plant in the world; or North Twin Peak Island Mallee, Eucalyptus insularis, which could not be more different from any other eucalypt with its dwarf mallee form, delicate leaves and exclusive granite rock habitat. All three species belong to the monocalypts which, as a group, are regarded as the most derived and modified of all the eucalypts.

The basis of classification

To seek a classification of the whole genus, one begins with what appear to be the logical groupings of species, i.e. those that have in common many characters of high reliability in as much as they vary little, or not at all, within a species. A general rule is that the major divisions are based on the so far observed fact that species from one group cannot hybridize with one from another group. There is no absolute certainty in this concept and it is likely that research procedures will one day overcome the barriers between the groups. But this possibility should be no impediment to the intellectual construction of classifications of the species as they appear to the observer.

The first comprehensive classification of the eucalypts was published in 1934 by W.F. Blakely of the Botanic Gardens in Sydney. It treated 606 species and varieties and was based on the earlier researches of J.H. Maiden, a former Director of the Gardens who corresponded with the older Ferdinand von Mueller, the great botanical pioneer who worked in the Royal Botanic Gardens Melbourne. Blakely's classification remained the bible for Eucalyptus taxonomists for the next 37 years when a new but informal classification was published by L.D. Pryor of the Australian National University and L.A.S. Johnson of the Royal Botanic Gardens Sydney. In this latter work the genus was divided into seven subgenera. The whole scheme was a great advance on Blakely, but its informal nature precluded its general use in formal botanical literature.

Johnson in 1995 with the collaboration of K.D. Hill created a new milestone in Eucalyptus taxonomy when they 'split' the genus with the publication of a new genus, Corymbia. This new genus comprised the ghost gums and the bloodwoods. The remainder of the genus Eucalyptus remained untreated at this level in a formal taxonomic sense.

In the Australian National Herbarium, we have taken a more conservative approach. In doing so, we have retained both genera Eucalyptus and Angophora in the traditional sense. The 800 or so species of Eucalyptus are divided into 13 subgenera, two of which are the ghost gums (subgenus Blakella) and bloodwoods (subgenus Corymbia) that constitute Hill and Johnson's single genus Corymbia.

There are 13 subgenera in the concept of the whole genus Eucalyptus in this edition of EUCLID. Six of these subgenera consist of a single species only, the 'dead ends' referred to above. The remaining seven polytypic subgenera each consists of many species.This new formal classification of the genus Eucalyptus was published recently (Brooker, 2000).

The genus Angophora was published in 1797, within 10 years of Eucalyptus (1788). Various authors have considered it to be sufficiently distinctive that it should be maintained as a separate genus. Others believe it is a 'eucalypt'. As there is no definitive answer to date to this question, we have retained both generic names, as that is how the two groups are recognized by the general community.