Steve Jones - The Language of the Genes

Natural selection was Darwin's Big Idea. It gave him a mechanism that drove evolution without the need for a designer to supervise every step. The Origin of Species starts with life on the farm. It shows how domestic animals emerged from wild ancestors because of preferences, often inadvertent, for one type over another. Selective breeding, the choice of the best to produce the next generation, soon caused new forms to appear.

If farmers could do so much in a short time, then nature could do more. 'What limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure and habits of each creature favouring the good and rejecting the bad? I can see no limit to this power, in slowly and beautifully adapting each form li> the complex relations of life.'

The engine — il not the engineer-of evolutionary change is the preservation of favoured types in the struggle for life. Change is inevitable in any system, be it genes or language, which makes errors of transmission from one generation to the next. This may be evolution, but it is change at random. It cannot lead to the passage from simple to complicated which made humans from their modest predecessors. Natural selection takes advantage of the fact that, each generation, inheritance makes mistakes. Because some improve the ability of their carriers to cope with what nature throws them they copy themselves more successfully. Darwin's mechanism sorts out the best from what accident supplies. It gives a direction to evolution and allows life to escape from the inevitability of extinction. This ts as true for humans as for any other creature.

Selection is a simple idea. The notion is used by computer experts. They programme their toy not with the precise details of what is needed, but with a guess of what might work; and allow this to make rough copies of itself. By choosing the most successful, they make rapid progress and can, in a few generations, evolve computer birds that flock like starlings, mathematical ants able to follow trails and programmed flowers as beautiful and unexpected as any product of nature. Literature, too, is not immune to artificial natural selection. Just a few unpretentious themes underlie simple works like mediaeval folk tales or children's stories. To feed them into a computer with a small change to each motif and to choose the best leads to the emergence of brand new and coherent versions.

Humans are not safe from the Darwinian machinery. For most of history, most people died before they were old enough to pass on their genes. Even among the survivors, some had more children and some fewer. If any of these differences arc influenced by inheritance, then Darwin's mechanism is at work and the next generation will differ from its parents. Selection will, in time, lead to change.

The power of the evolutionary machine rests on its ability to choose the best available, even it it is nor much better than what went before. Lewis Carroll saw how it works. Imagine that we have a three-letter word — lpig' for example — and we want to change it into another — 'sty'. We can change any letter into any other. U we make random changes and just hope for the best, taking any meaningless set of letters each time, it takes thousands of moves to get the pig into the sty. Natural selection imposes a rule: all the words in between must make sense. It picks up combinations that look good and builds on them. It can get there in just six steps — pig, wig, wag, way, say, sty.

The theory of evolution caused a sensation in 1859, the year of publication of The Origin, because it seemed to remove the need for a direct link between god and man. The wife of the Bishop of Worcester said of it: 'Let us hope that it is not true — but, if it is, let us pray that it does not become generally known!' After the establishment had recovered from the shock, religious thinkers came up with the idea that evolution was a means to work out God's plan. Even if humans were not perfect, they were perfectible, and selection was how the deity had chosen to do it. However, its action, far from perfecting the imperfect, often seems incompetent or even cruel. Panglossians can find little comfort here.

Selection can do remarkable things. But much is beyond it. Natural selection cannot plan ahead; it acts, without foresight, taking no thought for the morrow. It does just what is needed and no more, and does it in a slapdash and shortsighted way. It is, to use Richard Dawkins' memorable phrase, a blind watchmaker, achieving an extraordinary end through a simple and inefficient means.

It is easy to claim the whole of biology as evidence of its action. Just as Paley interpreted the complexity of plants and animals as an argument for God, a neo-Paleyism plagues evolutionary theory. It claims that all animal structure is well adapted and must hence always reflect the action of selection. This argument is circular, but is hard to disprove. It has led to many disagreements — fascinating to their proponents, tedious to those outside — among biologists. Some feel that the Darwinian machine drives the whole of evolution from the shape of the nose to the order of bases in the DNA. Others see selection as an occasional event that directs some genes while most change at random. The issue is unresolved and some biologists like to spend their time making up stories about how selection has moulded the most improbable characters. Sometimes they even turn out to be right. Anthropologists have even more vivid imaginations and have made some unlikely guesses about how selection may have formed human attributes. Many are fantasy, but because they invoke events that happened long ago are almost impossible to refute.

Differential survival and reproduction are at work in many places without anybody realising. One egg in thousands and one sperm in millions produce an offspring. Do the rest die at random or do they fail for genetic reasons? Nobody knows: but if only the best survive, the Darwinian machine is a more pervasive force than was ever imagined.

Whatever its importance, selection is just a mechanism and not a force for good. Cancer patients are sometimes given a drug which attacks cells as they divide. The treatment often fails because selection is at work. A few cells have undergone a mutation which changes the properties of a certain gene to enable it to break down the drug.

These soon take over, sometimes so effectively that the patient dies. There is not much evidence of a benign designer here.

Humans show as well as anything else its strengths and weaknesses. Homo sapiens has changed as he filled the world over the past 150,000 years. Thar six thousand human generations is the same as the number of generations of mice that separate today's animals from those that infested the brand new Acropolis and ties modern fruit flies to the insects that swarmed over the apples of William the Conqueror. As far as anyone knows, today's mice and fruit flies have scarcely changed over that time, emphasising just how short a period we have had to evolve.

Three ages of history have moulded natural selection. A lengthy Age of Disaster was followed by a shorter Age of Disease and — very recently — by an Age of Decay. For most of the past nearly all those born died by disaster, through cold, hunger or violence. Many individual tragedies acted as agents of progress. This chapter is about how humanity evolved to cope with the changes in climate and diet as we moved from our African home. The second epoch, the Age of Disease (which began only a few thousand years ago), albeit over in the West, is still the rule elsewhere. The Age of Decay (in which most people die of old age) is now upon us. Because most of those who succumb nowadays have passed on their genes it is hard to know what selection will be able to achieve in this, the third age of human evolution.