These sea squirts were created when two evolutionary lineages fused (Image: Gary Bell / Taxi / Getty)

LOOK at the genome of a sea squirt and you’ll get a nasty surprise. Half of its genes have a straightforward evolutionary history. In fairness, so does the other half. Trouble is, the two histories are completely different. It seems that sea squirts do not, as we had thought, sit among the chordates, on the same evolutionary line as humans and other vertebrates. Instead, they are the result of what happens when you fuse an ancient chordate with the ancestor of a sea urchin.

The fusion of two distinct evolutionary lines is not supposed to work. According to received biological wisdom, any chimeras that result are meant to be evolutionary dead ends. Not for the first time, received wisdom appears to be wrong (New Scientist, 16 June 2007, p 48).

“There was a view that hybridisation was bad, and ‘pure’ species were good,” says James Mallet of University College London. The truth is, hybridisation is like mutation – mostly it’s bad, but occasionally it throws out something that meets a need. “Natural selection can use whatever inherited variation comes its way,” Mallet says.


Biologists are now coming round to the idea that much of nature is not a product of neat family lines, but a messy mass of cross links. That may be how, for instance, genetic instructions can turn something like a caterpillar into something completely different: a butterfly.

Such metamorphosis is particularly prevalent in marine life, perhaps because fertilisation takes place in an environment where sperm can sometimes be carried to the “wrong” egg. With so many different creatures leaving eggs lying around waiting to be fertilised, perhaps it is not surprising that an occasional misfertilisation takes hold and produces something totally unexpected.

The classic case is the starfish Luidia sarsi. It starts out as a small larva with an even smaller starfish inside. Eventually, the starfish moves to the outside of the larva. Then they go their separate ways. What started as one rather odd organism continues and ends life as two.

So, how widespread is this new biological mess? We know that probably at least 10 per cent of plant species were formed by some kind of hybridisation. The jury is still out on how important the process is in “higher” species. “We are only beginning to scratch the surface,” says Mallet.

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