I don't know how many of you have noticed a peculiarity of the 3
main Drosera groups in Australia: good vegetative reproduction! The
pygmies with their gemmae are the best example, followed by the tuberous
Drosera which are often found in large colonies resulting mainly from the
production of extra tubers, and then there are the petiolaris-complex plants
which multiply vegetatively by simple crown division.
It is accepted that vegetative reproduction was very important in
the evolution of the angiosperms since this is an excellent way to
establish new karyotypes resulting from chromosome fusion, fission, loss,
multiplication, etc. As you probably know, when a new karyotype arises
by chance in an individual of a population, the resulting hybrids between
this plant and one with the normal karyotype will very often be steril or
have low fertility due to meiosis problems. If this karyotype is for
some reason more advantageous than the "wild" karyotype, it has a much
bigger chance of being established in that population (and becoming the
new "wild" karyotype") if it finds another one like it to cross with.
When that species has some form of assexual reproduction, then this new
karyotype can spread easily. That's probably why we see such
enormous variation in chromosome numbers among the members of these 3
groups of Drosera in Australia.
Now at what point these forms of vegetative spread were developed
in these 3 groups and how much they are responsible for the speciation
process in Australia is pure speculation. Maybe the ancestors of
these 3 groups developed good means of assexual reproduction before they
adapted to the dry climate. Of course the tubers weren't present yet and
maybe the ancestors of the tuberous Drosera had thick roots like D.binata
(which I believe in nature also function as a means of assexual
reproduction like in cultivation). The gemmae probably weren't as
elaborate back then either.
Here in Brazil vegetative reproduction is not a strong point
among most Drosera species. I've never seen it in annuals, but have
observed it occasionally in most perennials. I sometimes see leaves near
the wet soil producing small plantlets in D.communis, D.montana, and
D.villosa. D.villosa often goes crestate in the wild (producing many
separate plants in the long run) and also rarely undergoes crown
divisions. Assexual reproduction seems to be strongest in D.chrysolepis
and D.graminifolia. When burned in dry season wildfires, new plants will
always emerge from the roots or base of the stems, often forming small
groups of plants originating from a single clone. Since this form of
reproduction is also present here in Brazil, as it most likely is in South
Africa (I'm not sure how those species behave in nature, if their fleshy
roots often give rise to new plants or not), not forgetting D.binata and
D.adelae in Australia, I believe that vegetative reproduction was
merely a co-factor in the speciation process of aussie Drosera, like the
water stress. Thus if both water stress and vegetative reproduction
are present outside Australia, that means we're still missing one or more
crucial co-factors responsible for the Drosera speciation process
in Australia
You mentioned that Drosera were most likely widespread in
Australia before the climate got drier, and that later on were reduced to
the SW, SE, and northern regions of the country. I believe the
concentration of species in SW Australia shows that the real "boom" in
speciation occurred in that region when it was already partially or
totally isolated from the other two regions. The pygmy and tuberous
species found in the SE would then be the result of migrations or are
simply descendents of the few species which had evolved while the SW and
SE were still connected.
The north and south could've been the first regions to be
separated by deserts, since we don't find pygmies nor tuberous species
in the north, meaning these could've evolved after the isolation of the
2 regions. It's funny to observe how similar D.banksii and D.subhirtella
are with the tuberous Drosera and how similar pygmy species are with the
petiolaris-complex Drosera, similarities which are unique characteristics
in the genus. D.banksii and D.subhirtella look basically like tuberous
Drosera without tubers while most of the petiolaris-complex species, with
their long petioles and small, rounded lamina resemble large pygmy
Drosera. Is this coincidence or maybe an ecological convergence as the
one observed in the traps of Sarracenia, Nepenthes, and Cephalotus to fit
similar necessities? I believe the morphological similarities show their
common ancestry, though what these ancestral plants looked like before
they were isolated in the north or south and evolved in their separate
directions, who knows?!
To finish off with some hope, an Australian CP'er once told me
that the soils in W.A. are slightly more radioactive than normal and that
this could've been responsible for the speciation process we're now
discussing. The extra radioactivity could've caused an abnormally higher
number of mutations. Mutations are most often bad for the plants, but the
more mutations occurring, the more chances you'll have of getting useful
mutations (though of course there's a limit to the mutations organisms
can take without croaking!). This radioactivity, if it exists and if it
really can influence speciation, could be the missing factor we're
looking for. The species would then be radiating out from W.A., where the
radioactive soils are present, which was most likely much easier back
when the deserts still had not split up Drosera into their 3 Australian
domains. This might even help explain odd-ball monsters like Cephalotus,
D.hamiltonii, tuberous Utrics, and the ex-Polypompholyx (now a subgenus
of Utricularia).
FERNANDO RIVADAVIA
Sao Paulo, Brazil