The Strange Inevitability of Evolution
Good solutions to biology’s problems are astonishingly plentiful.
BY PHILIP BALL / ILLUSTRATION BY JON HAN
Is the natural world creative? Just take a look around it. Look at the brilliant plumage of tropical birds, the diverse pattern and shape of leaves, the cunning stratagems of microbes, the dazzling profusion of climbing, crawling, flying, swimming things. Look at the “grandeur” of life, the “endless forms most beautiful and most wonderful,” as Darwin put it. Isn’t that enough to persuade you?
Ah, but isn’t all this wonder simply the product of the blind fumbling of Darwinian evolution, that mindless machine which takes random variation and sieves it by natural selection? Well, not quite. You don’t have to be a benighted creationist, nor even a believer in divine providence, to argue that Darwin’s astonishing theory doesn’t fully explain why nature is so marvelously, endlessly inventive. “Darwin’s theory surely is the most important intellectual achievement of his time, perhaps of all time,” says evolutionary biologist Andreas Wagner of the University of Zurich. “But the biggest mystery about evolution eluded his theory. And he couldn’t even get close to solving it.”
What Wagner is talking about is how evolution innovates: as he puts it, “how the living world creates.” Natural selection supplies an incredibly powerful way of pruning variation into effective solutions to the challenges of the environment. But it can’t explain where all that variation came from. As the biologist Hugo de Vries wrote in 1905, “natural selection may explain the survival of the fittest, but it cannot explain the arrival of the fittest.” Over the past several years, Wagner and a handful of others have been starting to understand the origins of evolutionary innovation. Thanks to their findings so far, we can now see not only how Darwinian evolution works but why it works: what makes it possible.
A popular misconception is that all it takes for evolution to do something new is a random mutation of a gene—a mistake made as the gene is copied from one generation to the next, say. Most such mutations make things worse—the trait encoded by the gene is less effective for survival—and some are simply fatal. But once in a blue moon (the argument goes) a mutation will enhance the trait, and the greater survival prospects of the lucky recipient will spread that beneficial mutation through the population.
The trouble is that traits don’t in general map so neatly onto genes: They arise from interactions between many genes that regulate one another’s activity in complex networks, or “gene circuits.” No matter, you might think: Evolution has plenty of time, and it will find the “good” gene circuits eventually. But the math says otherwise.
Take, for example, the discovery within the field of evolutionary developmental biology that the different body plans of many complex organisms, including us, arise not from different genes but from different networks of gene interaction and expression in the same basic circuit, called the Hox gene circuit. To get from a snake to a human, you don’t need a bunch of completely different genes, but just a different pattern of wiring in essentially the same kind of Hox gene circuit. For these two vertebrates there are around 40 genes in the circuit. If you take account of the different ways that these genes might regulate one another (for example, by activation or suppression), you find that the number of possible circuits is more than 10700. That’s a lot, lot more than the number of fundamental particles in the observable universe. What, then, are the chances of evolution finding its way blindly to the viable “snake” or “human” traits (or phenotypes) for the Hox gene circuit? How on earth did evolution manage to rewire the Hox network of a Cambrian fish to create us?