Re: _Drosera_ evolution

From: SCHLAUER@chemie.uni-wuerzburg.de
Date: Mon Nov 02 1998 - 17:07:13 PST


Date:          Mon, 2 Nov 1998 17:07:13 
From: SCHLAUER@chemie.uni-wuerzburg.de
To: cp@opus.hpl.hp.com
Message-Id: <aabcdefg3476$foo@default>
Subject:       Re: _Drosera_ evolution

Dear David,

> I'm curious about the evolution of Drosera. They are certainly almost
> worldwide; however, some continents, Australia for instance, seem to have
> larger numbers than others. Does that mean that Drosera originated in
> Australia when it was part of the larger land mass that once existed.

Perhaps some of the older discussions on the same topic (see archives
of this list) are of interest to you.

Species numbers alone can be misleading. You have to look at the kind
of species and the range of diversity. In Australia you will, e.g.
find most of the _Drosera_ species to belong to the endemic or almost
endemic subgenera Bryastrum (pygmy sundews) and Ergaleium (cormous
sundews). Chances are good that most of the extant species in these
groups originated rather "recently" as the product of adaptive
radiation in the areas that are (probably) secondary, recent centres
of diversity. But this does not mean that Australia is not the place
of origin of the genus. In fact the high number of endemic or
subendemic sections and subgenera present in Australia (Stelogyne,
Lamprolepis, Coelophylla, Bryastrum, Phycopsis, Ergaleium) is a good
indication for the hypothesis of an Australian origin of _Drosera_.
At least the genus must have been there for a very long time.

The fossil record is not at all conclusive. The oldest known
(suspected) _Drosera_ pollen is from the European Tertiary, which
does not mean that the genus originated here but rather that there
have been a rather large number of palaeontological studies in
Europe compared to other regions. The old age of Droseraceae as a
whole is documented by the fact that there are fossil seeds of
_Aldrovanda_ from the Upper Cretaceous (approx. 70 million years
before present). The distribution of fossil Droseraceae pollen
indicates that the family disappeared from many regions where it
occurred previously. It is most likely that some genera did exist
that are now extinct. Fossil _Dionaea_-like pollen is known from
Europe.

At least it can be stated that diversification in Australia was of
greatest relevance to the present situation in _Drosera_.

Kind regards
Jan



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