Rhipsalis

[DaveW]

I was searching for some information on Rhipsalis on the Internet and found this article that may be of interest.

http://rhipsalis.com/maxwell.htm

[DaveW]

The question often arises for all these theories is Rhipsalis an old genus or one recently evolved?

Also see:-

http://www.pnas.org/content/108/20/8379 ... nsion.html

http://hss.ulb.uni-bonn.de/2011/2703/2703-1.pdf

[KarlR]

I think the question of just how Rhipsalis ended up in the Old World is a very interesting one. It seems unrealistic that it should have been spread by man. And the idea that it should have been originally spread by a bird or travelled on a raft seems very questionable. On the other hand, I find the theory proposed by Maxwell (in the first article you linked to) far more unrealistic. He seems to contradict himself on more than one occasion and the overall impression I'm left with is someone who is very much trying his hardest to fit the evidence to his theory.

Recent research (as in the Arikaki paper and e.g. in Schumannia nr. 7) mostly seem to agree on a much more recent origin of the Cactaceae some 30 million years ago or thereabouts. A complete lack of fossil evidence makes things difficult, but the results of these studies seem quite solid. Just as a suspension of disbelief is perhaps required in the "spread by birds" theory, it certainly requires a hefty dose of the same to imagine that the family developed before Africa split from South America and that the only species which spread into Africa (while the continents were still tied together) was Rhipsalis baccifera (or an immediate ancestor). And, furthermore, a suspension of disbelief is most certainly required to accept that over the 130 million years R. baccifera would then have existed in the Old World, it has not evolved at all (considering the same species exists in the New World). It is simply such a stretch of the imagination that for me it completely undermines any of Maxwell's other arguments.

I suppose it's possible that the species might have originally been brought to the Old World by sailors, and subsequently been transported further by birds. It seems extraordinary that it should have attained such a wide distribution based on this, although an W-E distribution across Africa by birds and/or bats is probably fairly plausible. Even though there are some serious question marks behind such a theory, I certainly find it more plausible than R. baccifera basically being a "living fossil", not having evolved (on either continent) for ca. 130 million years.

As far as I'm concerned, all the evidence points against an origin more than 130 million years ago, and so the question of how R. baccifera spread to the Old World is basically whether the initial spread was by man or by bird (or possibly by raft).

[D^L]

Were this evidence that cacti evolved when the continents were joined, then Rhipsalis baccifera, present on both continents, must be basal in the cacti. Both dna studies and older assessments agree this is highly unlikely. The balence of evidence is strongly in favour of transport across the ocean.

All of the other theories require us to believe this was driven by a very rare event but I agree this is much more likely than assuming everything we know about evolutionary direction is wrong.

R. baccifera is one of the species most evolved for long distance distribution, as a result of sticky material round the seeds. It seems a huge co-incidence if this species was the most distributed round the world BUT by a different mechanism. My best image is of a bird, with seeds stuck to beak and feathers, blown accidentally to Africa.

Interestingly there are at least 2 other 'American' families that have one representative in Africa. The bromeliads is one the other is related to these but I don't recall its name.
Cheers
David Lambie

[D^L]

There is another interesting paper by Barthlott looking at herbarium specimems over the last couple of hundred years. This shows that the known distribution has not changed much over a couple of hundred years.

This suggests that the transfer was not very recent - at least long enough ago that the distribution has stabalised.

Cheers
Davis Lambie

[Ali Baba]

Interestingly there are at least 2 other 'American' families that have one representative in Africa. The bromeliads is one the other is related to these but I don't recall its name.

Pitcairnea and Maschalocephalus

[D^L]

Thanks for the names! Bromeliaceae and Rapateaceae. I know nothing of the latter except in relation to the member in Africa.
Cheers
David Lambie