From angelic sex to sinful genes

Publishers' and readers' fascination with genes and genomes helps maintain the current boom in popular science. These two worthy books feed that fascination, but in ways that also undermine simple ideas about the power of genes. Although both authors spend most of their working lives trying to figure out how genes do their stuff, they also, in different ways, see reason to cut genes down to size. Genes are important players in life's game but - says Mark Ridley - they do not make the rules, nor - says John Medina - are they the only players.

Ridley, from Oxford's zoology department, is often confused with the science journalist and acclaimed popular biology author Matt Ridley. Mark Ridley's first trade book will do nothing to dispel the confusion. He joins a recognisable school of popular writing about evolutionary theory that includes Matt Ridley as well as its founder, Richard Dawkins. All are offering ever more detailed answers to the question of how individual genes, competing among themselves for a share of the next generation's DNA, can account for all the other peculiar features of life on earth.

Popular science is often taken to be about explanation. True, but quite often, too, the reader has to be persuaded that something needs explaining. So it is here, and Ridley devotes several chapters to outlining his key evolutionary problem. It is not, as once widely believed, the origin of life. That now looks like it happened pretty quickly, geologically speaking.

The emergence of complex forms, though, took much longer and is harder to account for. Some simple creatures still live happily much as they did when they emerged from the primeval slime. So why did others ever crawl further or strive to get more interesting? And how did they overcome the obstacles to maintaining complexity that are now clearly apparent?

The difficulty lies in the DNA. Natural selection needs variation in the candidates for survival in the next generation, but not too much. Copy a long stretch of DNA even once and the result will be riddled with mistakes. (Try typing out a page of The THES over and over again and you will see the point.) Follow a gene down the generations and you have a never-ending game of Chinese whispers. In DNA replication, the incidence of misread bases means that human beings make about 200 copying mistakes every time a child is produced. In theory, this is 199 too many to avoid the species succumbing to a mutational meltdown.

So the evolution of complexity is partly the story of how mechanisms emerged for coping with "lo-fi" copying of larger numbers of genes, with their larger amounts of DNA. How to avoid an error catastrophe? DNA proof-reading and repair are part of the answer. So, it turns out, is sex. Mingling the genes from two individuals means that errors will be redistributed and at least some offspring will emerge error-free. The rest can then be removed, along with their dud DNA, and the species can endure. Then, to prevent subversion of this safeguard by selfish genes, there is the separate internal gene shuffling known as recombination. This is the prompt for Ridley's subtitle. Gene justice exists in that each gene has precisely a 50 per cent chance of being passed on. Recombination prevents any gene in a sexually reproducing organism from increasing its chances above 50 per cent by beating up on what is, in effect, its competitor in a matched pair of chromosomes.

The appeal of this as a popular exposition is, I suppose, the way a few relatively simple principles can be extended to explain lots of apparently different things. Pretty similar, then, to its appeal as science. As John Maynard Smith points out from time to time, the trouble with biology is there are too many damn facts. Ridley, like Dawkins, can make lots of those facts line up in an orderly way. The phenomenal levels of foetal-manufactured hormones in the blood of pregnant women become part of his scheme. So, most gratifyingly for me, does the otherwise ridiculous matter of meiosis, in which in order to halve the number of chromosomes the cell first doubles them. It is all to do with disguising the ultimate destination of individual genes.

Ridley's book follows a pretty standard structure. First, establish a problem. Then, explain a range of solutions. Next, show how all those solutions can be described in the language of gene replication and competition. Finish with a little speculation, in this case about whether the complexity of living things has reached its limits, or could ever increase (cue some entertaining thoughts about how elaborate sex between angels might need to be). The structure works well because Ridley has a single explanatory principle that plays out in complex ways.

Medina, a molecular geneticist at the University of Washington, has a different narrative problem. He is interested in behaviour, hence in brains as well as in genes. He has plenty of those damn facts, but part of his point is that there is no overarching explanation, certainly not a genetic one. His solution is to build up his picture of the complexities of biology and behaviour by repetition. Each chapter considers one kind of behaviour, looks at what we know about its cellular basis, especially in the brain, relates any known genetic effects, and closes with a reflection on what all this tells us about minds and brains, emotion and consciousness. The whole thing is a rich introduction to the current state of the art in understanding the interplay of genes, hormones, synapses and neuro-transmitters, which somehow link brains and behaviours.

The novelty is how he prefaces each excursion into the world of research results, which are often confusing or even downright contradictory. They all begin with a vignette from the "Purgatory" section of Dante's Divine Comedy . The behaviours that Dante's sinners are being so imaginatively punished for - the seven deadly sins - are the ones Medina tries to explain. Well, not quite. For his clever interweaving of Dante with modern science has mixed motives. One, as he declares at the outset, is to signal that this is a book for literary types - no prior knowledge required. Another, which emerges more gradually, is to emphasise the distance between biological understanding and our everyday notions of behaviour, which are not so different from Dante's. More subtly, the constant reference to a writer with a profound understanding of human nature throws into relief the strengths and limitations of a more scientific approach.

So, while Medina's chapter titles follow the seven sins, it becomes apparent that several, such as avarice, envy and pride, have no obvious neuro-biological correlates. He quickly ends up discussing fear, depression and learning instead. All eminently researchable topics, but the substitutions underline the limits of the explanations. The result is a cleverly constructed account of some of the most difficult problems of contemporary biology, enhanced by Medina's own rather good diagrams. It is more obviously didactic and less donnish than Ridley's volume, but no less enjoyable in its way. There are a good many bad books about genetics around, but these two are welcome additions to the popular literature.

Jon Turney teaches science communication in the department of science and technology studies, University College London.