What is convergence? Consider your eye and that of an octopus. Both are built based on the camera principle, yet you are closely related to a starfish while the octopus is a near cousin of the oyster. The common ancestor of you and the octopus lived about 550 million years ago and at most possessed a simple eye-spot. Regarding the eyes, vertebrates and molluscs have arrived at the same solution, and in doing so have solved equally successfully problems such as how to correct spherical aberration. Camera-eyes are a brilliant evolutionary invention, and so it is less surprising that they convergently emerged in at least five other groups, including cubozoan jellyfish. And here is something else these groups have in common: with the exception of some snails that are ‘landscape artists’ (well, they are adept at spotting routes to safety on salt-marshes), all are fast-moving, predators and show an interesting tendency towards intelligence.
But aren't all camera-eyes built using the famous Pax-6 gene? Indeed, but so are compound eyes and they evolved independently at least four times. Pax-6 was almost certainly recruited from a more primitive role in the development of anterior sensory fields. That explains why this gene is also expressed in the nose and brain, as well as salivary glands. Remember also that nematodes lack eyes, but they possess Pax-6. To insist that a gene like Pax-6 ‘makes’ an eye is an over-simplification: necessary but not sufficient.
Isn't convergence obvious? Tell that to the famous Victorian naturalist Henry Bates. In the Amazon, he was hunting hummingbirds and — extensive as his knowledge was — he routinely shot sphinx moths by mistake. Local people insisted that moth could transmute into bird; silly but understandable because the convergence is remarkable not only in terms of body shape and flight dynamics, but even in their similar energy budgets.
Did you say ‘remarkable’? Odd isn't it, but almost invariably when biologists describe convergence the words they use are: ‘striking’, ‘astonishing’, ‘stunning’, ‘uncanny’, and, yes, ‘surprising’. Why? Surely we all agree that the organisms must function, that physical laws apply, and that adaptation is real. Moreover, convergence is ubiquitous but it can be difficult to see the wood for the trees — oh yes, they are convergent, too. In these atomistic, reductionist and specialized times it is easy to forget that organisms — and cells — are functionally integrated. Look at the lamnid shark and tuna. Similar body shape, specialized muscle-tendon system and even warm bloodedness provide a wonderful example of convergence. The similarities are far more than skin-deep.
Do you ever see examples of molecular convergence? In principle, one shouldn't. Given the size of a typical protein and the 20 available amino acids the number of alternatives is more than astronomic. Nevertheless, molecular convergence is probably far more common than realized. Consider that chemically intractable molecule carbon dioxide, stable and with strong ionic bonds but the management of which is central to biological processes such as photosynthesis, biomineralization, and respiration. The key enzyme in CO2 metabolism is a metalloprotein, carbonic anhydrase. Evolved once, twice? No — at least five times independently. And that is modest when compared to C4 photosynthesis, which has arisen at least 30 times.
Do extraterrestrials see with camera-eyes and breathe using carbonic anhydrase? Almost certainly; they are the obvious solutions. Evolutionary convergence allows us to predict what, one day, we might encounter. Not only in terms of eyes, but other sensory systems such as echolocation which has evolved independently at least three times — in bats, whales and birds. So too, in terms of social systems, think of the colossal convergence between elephants and sperm whales. Then there is eusociality, a system that has evolved repeatedly in insects and moreover in shrimps and even mammals, in the case of the naked mole rats. The mole rat is one of the very few examples in biology where a system was predicted before it was actually recognized. So, our planet may actually provide a very good guide to alien biospheres. Even if the planetary environment is very different, say a very dense atmosphere or giant oceans, we can still make a good estimate of what one day we may find. Not only that, but convergence tells us aliens will even think in much the same way.
Now you are joking! Neo-Darwinians typically assume human-like intelligence is an evolutionary fluke, a historical accident. If correct, then the Search for Extraterrestrial Intelligence (SETI) is a complete waste of time. But what we see on this planet tells us otherwise. All the evidence suggests the cognitive world of dolphins is remarkably similar to that of the the great apes. Certainly, both are mammals, but chimps don't live in oceans and the brain structures are markedly different. Even more remarkable is the cognitive architecture of birds, especially crows. Again it maps closely against the mind of the great apes but their brain is now known to be built to a completely different plan. When it comes to tool use the New Caledonian crows are well ahead of chimps. And the convergences don't stop there: warm-bloodedness and singing are convergent with mammals. So is social play. Did you know crows enjoy skiing? And what about the New Zealand parrot known as the kea? Watch out for those delinquent gangs of teenage birds as they roam around trashing cars.
Are we on the threshold of a general theory of evolution? Maybe.Why does convergence matter? It shows adaptation is real, and not some Darwinian conspiracy. It insists that organisms are functionally integrated and not a heap of character states. Paradoxically, the very similarities seen in convergence are some of the best proofs of evolution. Next time you are cornered by a pair of creationists order them a stiff gin and tonic and then ask him why the position of the retina is opposite in our eye to that of octopus (clue: embryology), and ask her why the bacterial flagellar motor has evolved at least twice. Then when they are sobering up remind them that the way in which Drosophila reacts to ethanol is remarkably similar in terms of behaviour to the manner in which we get drunk. Please raise a glass to convergence.
Donnerstag, 25. Januar 2007
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