by Matthew Cobb
It’s a notorious fact that there are no pictures in Darwin’s On the Origin of Species. Or rather, there’s a single diagram, showing how Darwin saw the evolution of species , with whole families (‘branches’) going extinct at irregular intervals of time.:
This wasn’t Darwin’s first stab at the question, though – in his notebooks during his voyage on the Beagle he drew a coral-shaped diagram and wrote “I think” by the side:
Since this time, there have been thousands of evolutionary trees drawn up. Unlike previous trees of life they are not simply about classifying organisms, they are also hypotheses about how related organisms are to each other.
For a long time, phylogenetic trees were the restricted domain of bearded professors arguing about the significance of the porpoise’s left thumb, or whatever. With the advent of molecular data and the growth in computing power, scientists have become very interested in them once again.
Part of the reason for this is that there is plenty of room for argument – something scientists put up there with ‘discovery’ on the scale of what’s really important. The thing is, evolution took place in just one way. There was just one true sequence of how the species on our planet split, evolved and died out. And given the number of species that have lived on the Earth over the last 4 billion years, there are more ways of arranging those species in a tree than there are atoms in the Universe. Really.
For example, if you have just 10 taxa (a neutral word that could mean a species or any group), there are about 3,600,000 possible trees. Because of the way the mathematics works, if you double the number of taxa you far more than double the number of possible trees – it ends up at 2,000,000,000,000,000,000 trees. And how many species have been around over the last 4 billion years? We have no idea – anywhere from a dozens of millions upwards.
Within those zillions of possible trees, there is just one that is true. No one imagines we’ll ever find out which one it is (how could we know?). All we can hope is to try and get the Big Picture right (are humans more related to goats or dogs?), and to fiddle around with the fine detail at the ends of the various branches (the relations between various modern species of fruitfly are a particular favorite).
I was prompted to write this because Current Biology, a fortnightly research journal, is about to publish a new tree of ‘deep animal relationships’, which tries to see how bilaterally symmetrical animals (like us) are related to sponges, and jellyfish.
Based on the sequence of 128 genes, the new tree suggests that the four kinds of sponge are a single group (they are ‘monophyletic’) – suggesting that the earliest animals were not sponge-like – and that the comb jellies (‘ctenophores’) belong with the jellyfish (‘cndarians’). What’s fascinating is that this arrangement is precisely that taught many years ago on the basis of comparative anatomy, and challenged by previous molecular studies…
Is the tree right? I have no idea!
Citation: Hervé Philippe et al. (2009) Phylogenomics Revives Traditional Views on Deep Animal Relationships. Current Biology – 02 April 2009
The article – which has not yet been published – can be found here, but you or your institution will need a subscription to read more than the abstract.