麻豆传媒

IMAGINE stumbling across a brass watch during a walk on the heath, reflected the English clergyman and philosopher William Paley back in 1802. Who could doubt that the watch had been designed by a creator? The same, he said, must be true of 鈥渁ll the works of nature鈥�. There must be a divine creator.

Half a century later, Darwin challenged this view. All the extraordinary diversity and complexity of life, he argued, could be explained by two processes 鈥� variation and natural selection. In any population of living creatures, there are small inheritable variations. The variation is random, but the chance of any variation being passed on is not. Harmful changes are weeded out by natural selection, while favourable ones spread rapidly through a population. Over time, single-celled organisms evolved into blue whales and giant redwoods.

But is this really plausible? Take the watch. Only a tiny change in the size of a gear could be tolerated without jamming the mechanism. Doubling the size of the watch by variation and selection would take aeons. By comparison, the 600 million years it has taken the first multicellular creatures to evolve into the incredible range of forms roaming Earth today seems short.

The answer, according Marc Kirschner of Harvard Medical School and John Gerhart of the University of California, Berkeley, is that variation is not random. Instead, it tends to favour changes that allow organisms to adapt. In The Plausibility of Life, they propose a new theory of evolution called 鈥渇acilitated variation鈥�.

At first glance, this smacks of Lamarckian heresy, of a hidden hand guiding evolution. How can variation possibly be facilitated? Because of the way biological systems work. Genome studies reveal that many of our genes are astonishingly similar to those in other organisms. For instance, some of the Hox genes that control our embryonic development are almost identical to those in flies.

These genes are conserved because they are involved in core processes: embryonic development, signalling between cells, metabolic processes and so on. New core processes appear only rarely in evolution, Kirschner and Gerhart argue, because they require whole sets of new proteins interacting in complex ways.

When a new process evolves, it is fragile: small mutations or changes in temperature may muck it up, for instance. But because there is strong selection for organisms that can adapt to a wide range of conditions, over time core processes becomes more robust and flexible.

Take the cytoskeleton within cells. New microtubules constantly form and grow in all directions. Some are stabilised, while others shrink. This process allows the cytoskeleton to form a vast variety of shapes as needed, rather than being limited to a predetermined form. Nerves and blood vessels develop in a similar way. Genes determine the major outline of the systems, but fine-tuning is achieved by exploratory growth, reinforcement of useful connections and the paring away of the rest. This plasticity allows our bodies to adapt to different environments.

Suppose a mutation makes the vertebrae in an animal鈥檚 neck grow longer. If animals were constructed like a watch, that change would be fatal. But thanks to the adaptability of core processes, the blood vessels, nerves and muscles all grow along with the neck bones. The evolution of the giraffe is not so implausible after all. This is Kirschner and Gerhart鈥檚 central argument. Genetic variation may be random. But the resulting phenotypic variation is not. By altering the processes that allow individuals to cope with changing conditions, mutations are more likely than chance alone to produce changes that make offspring more or less suited to particular environments.

According to facilitated variation, evolution proceeds in three stages. New core processes occasionally evolve. Over time, they become more robust and flexible. This makes other genetic changes, such as the one that resulted in a longer neck, less likely to be lethal. So organisms with very similar sets of genes can evolve into an immense variety of forms 鈥� rather like rearranging Lego blocks. The basic idea is that life has evolved to evolve.

Kirschner and Gerhart claim that facilitated variation is a 鈥渕ajor new scientific theory鈥�. It鈥檚 more hypothesis than theory. But their ideas are certainly challenging and provocative.