The orchid floras of Africa, Asia, the New World and perhaps Australia are extremely distinct from each other. This is strongly mirrored in other groups of plants. One obvious thought is that this is due to the break-up of the continents, with - for example - Lauresia splitting from Gondwanaland as the Atlantic opened up. A chunk that split off Africa dropped Madagascar, the the Seychelles (most of which sank) and ended up as India, butting into Asia with a freight of plant life. Many plants of Madagascar are intermediate between Asia and Africa, for example. Orchids such as Acampe rigida are found in all three places. Groups such as the Cymbidieae are present in all three, in distinct forms as they evolved apart. But an Ansellia is very close to a Gramatophyllum, for example; but with centres of radiation thousands of miles of ocean apart. .
Unhappily for this neat scheme, there are Cymbidieae in South America, and annoying genera - Bulbophyllum, Vanilla, Polystachia - that are globally distributed. (As are many temperate terrestrials - but these probably got distributed across the Northern hemisphere by land connections.) Quote, therefore: A new report published in the Journal of Biogeography has shown that, while continental segregation is probably the case for some of these plants, others are far too recent to have lived at the time when the super-continent broke up. They must therefore have dispersed across oceans to reach their current distribution ranges. Barker et al. apply a technique known as molecular dating to DNA sequences from over 40 representatives of Protaceae from all southern continents. Using carefully selected fossils that are of known age and affinity, the mutation rate of the DNA sequences was calculated, allowing these scientists to provide age estimates for evolutionary events in the family. "Our results show that ancestors of some of the modern Proteaceae must have crossed the Atlantic and Indian Oceans. Thus, in Africa, for example, the spectacular genus Protea is truly Gondwanan, but 250 species from other genera that occur in the 'fynbos' vegetation (literally, 'fine leaved shrubs') of the highly diverse south-western Cape biodiversity hotspot are much younger, and have Australian relatives" says Nigel Barker of Rhodes University, South Africa. This new finding is important, as it challenges the dogma that gondwanaland's biota merely moved in situ with the continents as they broke up. "We have to reconsider the possibility of transoceanic dispersal, as unlikely as it sounds for these plants" says Peter Weston, a researcher at the Royal Botanic Gardens, Sydney, Australia. ---end quote. One wonders how this actually happened: Protea branches bobbing in the ocean, and rooting on a foreign strand? It works for so-adapted palms and mangroves, but proteas? Or more likely, therefore, seeds ingested by a wandering, migrating bird? Applied to orchids, however, one truly wonders... Theoretically, an orchid seed could blow many tiems round the world before settling. But if so, and gioven the vast number of seeds that are dispersed, why is the worlkd not uniformly covered with them? Darwin's famous calculation about what would happen if all of the seeds from an Orchis plant were to germinate in ideal distribution: in the first generation, they would cover a large field, in the second, the Isle of Wight, in the third, they would carpet the whole planet. ______________________________ Oliver Sparrow +44 (0)1628 823187 www.chforum.org _______________________________________________ the OrchidGuide Digest (OGD) [email protected] http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
