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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.
References
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