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Out of Australia
Why do many Australian native plants have relatives growing in Africa, Asia or other geographically distant regions. How did they get there? These long-distance relationships have intrigued biologists for the last couple of centuries.
In the last few decades there has been fierce debate about whether such plants spread and diversified because of continental drift (meaning that the ancestors of current species evolved before the continents separated), or whether the species evolved on one continent after the continents had separated and then spread by long-distance dispersal of seeds or spores by wind, water or animal transport.
Recent molecular techniques are helping greatly to test such hypotheses, especially with increasing computing power to run complex analyses. And more fossils are being found, which give date estimates (couched in millions of years) as to when the ancestors of current species existed and what the ecological conditions may have been back then. There is now good evidence confirming that some flowering plant groups existed before the break-up of Gondwana but that many groups evolved after its break-up, meaning that long-distance dispersal is the only plausible explanation for their current distributions.
Long-held ideas have been turned on their head by recent studies. Perhaps the best example is the notion that all flowering plant groups originated in Africa and then they or their descendants spread to other landmasses. This seems to be only partly true, as shown by studies such as that of the family Restionaceae by Dr Barbara Briggs and colleagues (published in 2003) and in Proteaceae by Dr Peter Weston and colleagues (published in 2007). The Proteaceae study showed that the ancestor of Leucadendron and its closest relatives is most likely to have gone from Australia back to Africa (where another group of Proteaceae probably originated and persisted), so this was a case of 'Back to Africa'.
A recently published study of schoenoid sedges (tribe Schoeneae in family Cyperaceae) by Senior Research Scientist Karen Wilson and university colleagues in Armidale, Perth and Cape Town has come to an even more radical conclusion: that the schoenoid sedge tribe originated in Australia about 50 million years ago and then spread by long-distance dispersal to neighbours like New Zealand, New Guinea and New Caledonia and farther afield to Africa, Asia, the Americas and even Europe. Furthermore, their analyses suggest that this was not a single dispersal from Australia but an estimated 29 dispersals between the landmasses, starting with a dispersal from Australia to southern Africa 30-35 million years ago and the rest (in various directions) mostly in the last 20 million years.
The tribe Schoeneae seems to have evolved rapidly in Australia about 50 million years ago into six main lineages, all six still found in Australia and exemplified by the genera Oreobolus, Caustis, Lepidosperma, Tricostularia, Gahnia and Schoenus. The first genus is well known in alpine habitats and the rest in nutrient-poor habitats such as around Sydney and Perth. There is another concentration of related genera and species in the Cape Province of South Africa, while other species are scattered in New Zealand, New Caledonia and Asia.
Habitat preferences of present-day species were investigated in this study. Why? And why look at past climates and habitats as deduced from fossil records? One might intuitively expect that close neighbours would have more genera and species in common than distant regions. However, this study suggests otherwise: that the availability of suitable climates and habitats in each region over the last 35 million years or so have been as important as actual distance between the landmasses in determining how easily an immigrant plant has been able to establish itself and then diversify locally. The schoenoid tribe has about 450 species now, nearly all in the southern hemisphere; only two species of Schoenus have spread widely in the temperate northern hemisphere. This is thought to be because of the greater availability of suitable habitats in the southern hemisphere for this tribe during its evolution.