Steven Pinker's Life's designer

... Richard Dawkins has ventured that life, anywhere it is found in the universe, will be a product of Darwinian natural selection... It is in fact a straightforward consequence of the argument for the theory of natural selection. Natural selection is the only explanation we have of how complex life can evolve, putting aside the question of how it did evolve. ...

The theory of natural selection... has an odd status in modern intellectual life. Within its home discipline, it is indispensable, explaining thousands of discoveries in a coherent framework and constantly inspiring new ones. But outside its home, it is misunderstood and reviled. ... I want to spell out the case for this foundational idea: how it explains a key mystey that its alternatives cannot explain, how it has been verified in the lab and the field...

Natural selection has a special place in science because it alone explains what makes life special. Life fascinates us because of its adaptive complexity or complex design. Living things... fly, or swim, or see, or digest food, or catch prey, or manufacture honey or silk or wood or posion. These are rare accomplishments, beyond the means of... non-living things. ...

Rare accomplishments come from special structures. Animals can see... because animals have eyes, and eyes have precise arrangements of unusual materials capable of forming an image: a cornea that focuses light, a lens that adjust the focus to the object's depth, an iris that opens and closes to let in the right amount of light, a sphere of transparent jelly that maintains the eye's shape, a retina at the focal plane of the lens, muscles that aim the eyes up-and-down, side-to-side, and in-and-out, rods and cones that transduce light into neural signals, and more, all exquisitely shaped and arranged. ...

The eye has so many parts, arranged so precisely, that it appears to have been designed in advance with the goal of putting together something that sees. The same is true for our other organs. ... One of the reasons God was invented was to be the mind that formed and executed life's plans. The laws of the world work forwards, not backwards: rain causes the ground to be wet; the ground's benefiting from being wet cannot cause the rain. What else but the plans of God could effect the teleology (goal-directedness) of life on earth?

Darwin showed what else. He identified a forward-causation physical process that mimics the paradoxical appearance of backward causation or teleology. The trick is replication. A replicator is something that can make a copy of itself, with most of its traits duplicated in the copy, including the ability to replicate in turn. Consider two states of affairs, A and B. B can't cause A if A comes first. ...

But let's say that A causes B, and B in turn causes the protagonist of A to make a copy of itself - let's call it AA. AA looks just like A, so it appears as if B has caused A. But it hasn't; it has only caused AA, the copy of A. Suppose there are three animals, two with a cloudy lens, one with a clear lens. Having a clear lens (A) causes an eye to see well (B); seeing well causes the animal to reproduce by helping it avoid predators and find mates. The offspring (AA) have clear lenses and can see well, too. It looks as if the offspring have eyes so that they can see well (bad, teleological, backwards causation), but that's an illusion. The offspring have eyes because their parents' eyes did see well (good, ordinary, forward causation). Their eyes look like their parents' eyes, so it's easy to mistake what happened for backward causation.

There's more to an eye than a clear lens, but the special power of a replicator is that its copies can replicate, too. Consider what happens when the clear-lensed daughter of our hypothetical animal reproduces. Some of her offspring will have rounder eyeballs than others, and the round-eyed versions see better because the images are focused from center to edge. Better vision leads to better reproduction, and the next generation has both clear lenses and round eyeballs. They, too, are replicators, and the sharper-visioned of their offspring are more likely to leave a new generation with sharp vision, and so on. In every generation, the traits that lead to good vision are disproportionately passed down to the next generation. That is why a late generation of replicators will have traits that seem to have been designed by an intelligent engineer.

I have introduced Darwin's theory in an unorthodox way that highlights its extraordinary contribution: explaining the appearance of design without a designer, using ordinary forward causation as it applies to replicators. ... In the beginning was a replicator. This molecule or crystal was a product not of natural selection but of the laws of physics and chemistry. (If it were a product of selection, we would have an infinite regress.) Replicators are wont to multiply, and a single one multiplying unchecked would fill the universe with its great-great-great...-great-grandcopies. But replicators use up materials to make their copies and energy to power the replication. The world is finite, so the replicators will compete for its resources. Because no copying process is one hundred percent perfect, errors will crop up, and not all of the daughters will be exact duplicates. Most of the copying errors will be changes for the worse... But by dumb luck a few errors will be changes for the better, and the replicators will accummulate any subsequent errors that are changes for the better... The resulting replicator with its apparently well-engineered body is what we call an organism.

Natural selection is not the only process that changes organism over time. But it is the only process that seemingly designs organisms over time. Dawkins... reviewed every alternative to selection that has been proposed in the history of biology and showed that they are impotent to explain the signature of life, complex design.

The folk theory that organisms respond to an urge to unfold into more complex and adaptive forms obviously won't do. The urge - and more important, the power to achieve its ambitions - is a bit of magic that is left unexplained.

The two principles that have come to be associated with Darwin's predecessor Jean Baptiste Lamarck - use and disuse, and the inheritance of acquired characteristics - are also not up to the job. The problem goes beyond the many demonstrations that Lamarck was wrong in fact. (For example, if acquired traits really could be inherited, several hundred generations of circumcision should have caused Jewish boys today to be born without foreskins.) The deeper problem is that the theory would not be able to explain adaptive complexity even if it had turned out to be correct. First, using an organ does not, by itself, make the organ function better. The photons passing through a lens do not somehow wash it cear, and using a machine does not improve it but wears it out. ... The inheritance of acquired characteristics is even worse, for most acquired characteristics are cuts, scrapes, scars, decay, weathering, and other assaults by the pitiless world, not improvements. And even if a blow did lead to an improvement, it is mysterious how the size and shape of the helpful wound could be read off the affected flesh and encoded back into DNA instructions in the sperm or egg.

Yet another failed theory is the one that invokes the macromutation: a mammoth copying error that begets a new kind of adapted organism in one fell swoop. The problem here is that the laws of probability astronomically militate against a large random copying error creating a complex functioning organ like the eye out of homogeneous flesh. Small radom errors, in contrast, can make an organ a bit more like an eye, as in our example where an imaginable mutation might make a lens a tiny bit clearer or an eyeball a tiny bit rounder. Indeed, way before our scenario begins, a long sequence of small mutations must have accumulated to give the organism an eye at all. ... Each mutation was improbable, but not astronomically so. The entire sequence was not astronomically impossible because the mutations were not dealt all at once like a big gin rummy hand; each beneficial mutation was added to a set of prior ones that had been selected over the eons.

A fourth alternative is random genetic drift. Beneficial traits are beneficial only on average. Actual creatures suffer the slings and arrows of outrageous fortune. When the number of individuals in a generation is small enough, an advantageous trait can vanish if its bearers are unlucky, and a disadvantageous or neutral one can take over if its bearers are lucky. ... because of its randomness, random drift cannot explain the appearance of an improbable, useful trait like an ability to see or fly. The required organs need hundreds or thousands of parts to work, and the odds are astronomically stacked against the required genes accumulated by sheer chance.

Dawkins' argument... works against two subsequent challenges. One is a variant of Lamarckism called directed or adaptive mutation. Wouldn't it be nice if an organism could react to an environmental challenge with a slew of new mutations, and not wasteful, random ones, but mutations for traits that would allow it to cope? Of course it would be nice, and that's the problem - chemistry has no sense of niceness. The DNA inside the testes and ovaries cannot peer outside and considerately mutate to make fur when it's cold... or to put a lens in front of the retina as opposed to between the toes or inside the pancreas. That is why a cornerstone of evolutionary theory... is that mutations are indifferent overall to the benefits they confer on the organism. They cannot be adaptive in general, though of course a tiny few can be adaptive by chance. ...

The other challenge comes from the fans of a new field called the theory of complexity. The theory looks for mathematical principles of order underlying many complex systems: galaxies, crystals, weather systems, cells, organisms, brains, ecosystems, societies, and so on. Dozens of new books have applied these ideas to topics such as AIDS, urban decay, the Bosnian war, and, of course, the stock market. Stuart Kauffman, one of the movement's leaders, suggested that feats like self-organisation, order, stability, and coherence may be "an innate property of some complex systems". Evolution, he suggests, may be a "marriage of selection and self-organisation".

Complexity theory raises interesting issues. Natural selection presupposes that a replicator arose somehow, and complexity theory might help explain the "somehow". Complexity theory might also pitch in to explain other assumptions. Each body has to hang together long enough to function rather than fly apart or melt into a puddle. And for evolution to happen at all, mutations have to change a body enough to make a difference in its functioning but not so much as to bring it to a chaotic crash. If there are abstract principles that govern whether a web of interacting parts (molecules, genes, cells) has such properties, just as it works within other constraints of physics and mathematics like the Pythagorean theorem and the law of gravitation.

But many readers have gone much further and conclude that natural selection is now trivial or obsolete... [They] must have reasoned as follows: complexity has always been treated as a fingerprint of natural slection, but now it can be explained by complexity theory; therefore natural selection is obsolete. But the reasoning is based on a pun. The "complexity" that so impresses biologists is not just any old order or stability. Organisms are not just cohesive blobs or pretty spirals or orderly grids. They are machines, and their "complexity" is functional, adaptive design: complexity in the service of accomplishing some interesting outcome. ... No set of equations applicable to everything from galaxies to Bosnia can explain why teeth are found in the mouth rather than in the ear. ... Matter simply does not have an innate tendency to organise itself into broccoli, wombats, and ladybugs. Nature selection remains the only theory that explains how adaptive complexity, not just any old complexity, can arise, because it is the only nonmiraculous, forward-direction theory in which how well something works plays a causal role in how it came to be.

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Because there are no alternatives, we would almost have to accept natural selection as the explanation of life on this planet even if there were no evidence for it. Thankfully, the evidence is overwhelming. I don't just mean evidence that life evolved... but that it evolved by natural selection. Darwin himself pointed to the power of selective breeding, a direct analogue of natural selection, in shaping organisms. For example, the differences among dogs - Chihuahuas, greyhounds, Scotties, Saint Bernards, shar-peis - come from selective breeding of wolves for only a few thousand years. ...

Natural selection is also readily observable in the wild. In a classic example, the white peppered moth gave way in nineteenth-century Manchester to a dark mutant form after industrial soot covered the lichen on which the moth rested, making the white form conspicuous to bird. When air pollution laws lightened the lichen in the 1950s, the then-rare white form reasserted itself. There are many other examples, perhaps the most pleasing coming from the workd of Peter and Rosemary Grant. Darwin was inspired to the theory of natural selection in part by the thirteen species of finches on the Galapagos islands. They clearly were related to a species on the South American mainland, but differed from them and from one another. In particular, their beaks resembled different kinds of pliers. ... Darwin eventually reasoned that one kind of bird was blown to the islands and then differentiated into the thirteen species because of the demands of different ways of life on different parts of the islands... But he despaired of ever seeing natural selection happen in real time... The Grants painstakingly measured the size and toughness of the seeds in different parts of the Galapagos at different times of the year, the length of the finches' beaks, the time they took to crack the seeds, the number and ages of the finches in different parts of the islands, and so on - every variant relevant to natural selection. Their measurements showed the beaks evolving to track changes in the availability of different kinds of seeds, a frame-by-frame analysis of the movie that Darwin could only imagine. Selection in action is even more dramatic among faster-breeding organisms, as the world is discovering to its peril in the case of pesticide-resistant insects, drug-resistant bacteria and the AIDS virus in a single patient.

... Finally, two kinds of formal modeling have shown that natural selection can work. Mathematical proofs from population genetics show how genes combining according to Gregor Mendel's laws can change in frequency under the pressure of selection. These changes can occur impressively fast. If a mutant produces just 1 percent more offspring than its rivals, it can increase its representation in a population from 0.1 percent to 99.9 percent in just over four thousand generations. ...

More recently, computer simulations from the new field of Artifical Life have shown the power of natural selection to evolve organisms with complex adaptations. And what better demonstration than everyone's favourite example of a complex adaptation, the eye? The computer scientists Dan Nilsson and Susanne Pelger simulated a three-layer slab of virtual skin resembling a light-sensitive cells above it, and a layer of translucent cells forming a protective cover. The translucent cells coul undergo random mutations of their refractive index: their ability to bend light, which in real life often corresponds to density. All the cells could undergo small mutations affecting their size and thickness. In the simulation, the cells in the slab were allowed to mutate randomly, and after each round of mutation the program calculated the spatial resolution of an image projected onto the slab by a nearby object. If the bouts of mutation improved the resolution, the mutations were retained as the starting point for the next bout, as if the slab belonged to a lineage of organisms whose survival depended on reacting to looming predators. As in real evolution, there was no master plan or project scheduling. The organism could not put up with a less effective detector in the short run even if its patience would have been rewarded by the best conceivable detector in the long run. Every change it retained had to be an improvement.

Satisfyingly, the model evolved into a complex eye right on the computer screen. The slab indented and then deepened into a cup; the transparent layer thickened to fill the cup and bulged out to form a cornea. Inside the clear filling, a spherical lens with a higher refractive index emerged in just the right place, resembling in many subtle details the excellent optical design of a fish's eye. ...

- Extract from Steven Pinker's How the Mind Works