|Botanists getting serious, Tarangire June 2011. I think it's a grass.|
I was brought up as an ecologist understanding that biogeographical (bio - from biology, geographic, from geography of course - biogeography being the study of distributions of species) similarities between continents could usually be explained by a process known as vicariance. This idea essentially explains the distribution of related species by assuming that a common ancestor of the current species lived on a continent that then moved around through the process of continental drift. A typical example might be the distribution of Ratites - the group of large flightless birds that includes the ostrich. The closest relatives to the ostrich include the emu in Australia, the rhea of South America and the kiwis of New Zealand. Their distribution in these southern continents is explained by their shared ancestor living on the ancient continent of Gondwana, a single continent that eventually broke up (around 200 million years ago) to form the southern continents (plus India and the Arabian peninsular). Each fragment carried a population of this ancient ratites and today we see a distribution of birds across the southern hemisphere.
|Ostrich are ratites, a typical Gondwanan group with a distribution explained by vicariance|
This explanation of shared ancestry, each population of which floated off on it's own continent it the one that immediately springs to the minds of ecologists of my generation where confronted with similar species across southern continents. But the break-up of Gondwana took place around 200 million years ago - and that's a very long time for evolution to have been acting. Although most ratites are fairly similar and the shared ancestry immediately obvious (though check the kiwis!), they're actually very different and certainly not in the same genus. Now, it's important to remember that, unlike species, genera are not very well defined groups - rather they are a taxonomists attempt to identify common ancestors and group similar species - but whether we group 50 similar species into five genera in one family, or one large and diverse genus within a family is rather more arbitrary than the similar decisions about species (though even there it's actually surprisingly tricky!). So I already knew that the best predictor of how long ago the common ancestor of any particular genus lived is nothing to do with the variety within the species, but everything to do with the number of taxonomists that work on the group - the more taxonomists, the more genera, the more recent the common ancestor. So my first question was whether the common ancestor of these plant species really lived more than 200 million years ago and are just kept in the same genus because there's such a shortage of taxonomists. And I learnt that whilst my head has been full of other things, I've missed one of the biggest revolutions in biogeography of the last decade.
|Meru's forests were full of genera also found in South America (and Australia!) Erica have interesting distributions, but not in the New World.|