More W. Kirschner and John C. Gerhart

THE PLAUSIBILITY OF LIFE

Resolving Darwin's Dilemma

Critics of evolution, including advocates of "intelligent design", often point to the complexity of life and say that it could not have arisen by the progressive accumulation of small genetic changes, especially random changes. And they do seem to have a point. Even the vast stretches of geological time do not seem to have been long enough. Also, how would random genetic changes be coordinated to give rise to a new kind of organism, one that functioned properly in its environment?

The answer proposed by Kirschner and Gerhart is that this is not how evolution works. They put forward instead a theory which they call facilitated variation. This draws on new information about molecular biology that only became available at the end of the twentieth century.

One of the most surprising things about evolutionary biology is the extent to which what the authors call "core processes" have been conserved from the earliest times—in fact, probably from the pre-Cambrian. As an example, more than 50% of the metabolic enzymes in the bacterium E.coli are identical to those in humans.

There are several hundred "core processes" that generate the anatomy, physiology, and behaviour of an organism during its development. Some of these have been unchanged for hundreds of millions or even billions of years. Yet although they are so strongly conserved, the way in which they behave can be altered by signals that are often quite simple. A process may just have two states, on or off, and the signals merely switch it from one state to the other. Hence the presence or absence of these signals, and their timing, can produce large changes in the organism's phenotype.

An important concept here is that of "weak regulatory linkage"; the term refers to reconfigurability (the linkages are not fixed) and also to instability. Instability might seem to be a bad thing, but Kirschner and Gerhart think that it has important advantages biologically. It provides flexibility. Weak regulatory linkage is important in evolution, the authors believe, because it plays a large part in core processes. It is not necessary for genes to provide detailed instructions for how the organism should develop. Instead, they can modify the way in which the conserved core processes function during development.

The book provides several examples of how this works, which are explored in some detail. They include the arrangment of microtubules in eukaryotic cells, the distribution of whiskers on the faces of mice and their central connections, and the limb proportions of different vertebrates. (Incidentally, I learnt here that a normal mouse has precisely 33 whiskers on each side.) The role of Hox genes and other selector genes in embryology is also considered. The embryological compartments which they control are remarkably similar in humans, fish, birds, frogs, reptiles and even sea squirts.

Even if the theory described here is correct, deep questions remain. How did the core processes originally arise? What was the origin of the first prokaryotic cell? And, most difficult of all, how did life itself originate? For these matters we are reduced to speculation, but evolution since the Cambrian, Kirschner and Gerhart believe, "is supported by irrefutable evidence and a compelling fossil record."

This is an important book, not least because it provides a pretty comprehensive answer to criticisms of Darwinism based on its alleged inability to explain variation. Whether or not it is ultimately accepted, it does at least offer a plausible mechanism. "[T]here are no gaping holes in our theoretical understanding of evolution and no conspiracy of silence on scientific findings." Unfortunately, it will probably not be widely read by non-specialists. The authors would like it to be, and say that they have tried to avoid jargon (and a glossary is provided). However, it is not exactly popular science writing, and the authors acknowledge that close attention from the reader is required and "there is no short cut to understanding," In other words, you have to work to get a grasp of their ideas, but the journey is worth the effort.

3 February 2006