>* Perhaps Australian Drosera have developed greater pollinator specificity.
>That's one reason why there is such an incredible variety of new world
>Orchids, compared to the relative poverty of the African orchid flora.
>What pollinates Australian Drosera, and how restricted is the range of the
>pollinators?
Little is known (or at least published) about that. However, some AU (and
other) species do rarely or never produce seed, others are autogamous, and
still others are obviously apomictic. In all these cases pollinators can
only play a very minor role in speciation.
>* Is there a difference in the way most Australian Drosera disperse their
>seed compared to those in the rest of the world?
Not really. All species do have very fine, dry seed without noteworthy
animal contribution (no food offered) to their dispersal.
>Do Brasilian drosera, D.
>rotundifolia and other widespread species rely on transport by birds?
Of course it is close to impossible to exclude this possibility. But it
does and did quite certainly not play an important role.
>If
>Australian Drosera rely only upon mechanical processes like rain, it could
>be that specific species might evolve in isolated drainage basins.
I would assume that rain is not in the first line a mechanical process here
because humidity is indeed a limiting vital factor for _Drosera_.
>Restricted seed dispersal is one explanation for the diversity of Lithops
>and other Azioaceae in South Africa, and might make sense for Drosera as
>well.
If seed is produced at all, it is certainly not restricted in terms of
dispersal comparable in any way to Aizoaceae (cf. the completely different
morphology, amount, and mode of dispersal).
>I'm surprised that Drosera is that old!
Why? As I have exemplified, it is probably even older than this.
> Over a period of 65 million years,
>I suspect the climate in both Brasil and Australia has undergone some
>periods of radical climate change.
Yes, but have a brief look at the relative sizes of both continents, and
keep in mind that AU has moved N considerably, while S AM +/- stood still
all the time.
> Does anybody out there know whether
>western Australia experienced a more disruptive climate over the last few
>million years than eastern Australia?
Magnus has answered this question already (but the last few million years
were supposedly not decisive for _D._ evolution in AU).
E AU has the pronounced advantage (in terms of humidity) to be in the E.
This sounds trivial, but in the very position which AU entered during
Tertiary (coming to cross the S tropic), this is indeed critical. Global
air circulation leads to formation of very dry and hot coastal deserts in
the W part of all continents at these latitudes (Baja California in N AM, W
Sahara in N AF, Atacama in S AM, Namib in S AF, and Eremaea in AU). Of
course it can be assumed that global climate has changed during Tertiary.
But in the trend, these regions can be assumed to have always been drier
than the adjoining ones (because the shape and rotation of earth remained
+/- the same).
> Have either experienced more
>disruptive climate changes than dry/wet periods in South America?
Certainly (again: AU has moved N, S AM not).
>if things were reversed and there were more Drosera species in Brasil, one
>might be able to construct a climatic rationale for hyperspeciation there.
If _Drosera_ was an Aizoacea, pollinated only by specific orchid
pollinators, distributed by birds, and washed away by the rain, inhabiting
all the great deserts on this planet (or even the oceans?), I bet nobody
would recognize this genus at all (;-)). As always in history, also in
natural history the question of alternative developments is rather futile.
But who knows?
Kind regards
Jan