9 April 2009

The end of evolution?

Cosmos Magazine
Humans are undergoing a 'grand averaging', argues British geneticist Steve Jones, because the raw material for evolution has largely disappeared.
Human diversity

Credit: iStockphoto

The word ‘utopia’ was coined by Thomas More, an English philosopher who had a vision of a future in which everything was perfect. The people living in his utopia were just like you and I, but they were living a new, magical and socially just existence.

A little more than 100 years ago, though, there was an abrupt change in the nature of our vision of the future. The people living in such fictional societies had changed – and the alterations were physical, not just social or cognitive. You can trace that shift to the beginning of evolutionary thinking in both the modern novel and in modern biology.

H.G. Wells wrote the first truly modern dystopian novel, The Time Machine, in 1897. A time traveller heads into the future and encounters what appears to be utopia. But slowly a terrible truth begins to dawn: the main characters, tea-drinking liberals called the Eloi, are regularly attacked by a terrible mob known as the Morlocks.

The Morlocks are sinister and brutal spider-like hominoids that live underground and come out at night to kill and eat the Eloi. The twist to the tale is that the Eloi are in fact the farm animals of the Morlocks.

The idea comes from an evolutionary concept that was very popular in Victorian days – that the human race was going to split into two types: the subhumans and the superhumans. In the Wellsian vision, human evolution has continued, bad genes have taken over, and we are in a state of inevitable physical decline.

Whether you subscribe to this view or to the more Star Trek style idea that people will become more advanced than they are now, both extremes hinge on the premise that evolution in humans will continue.

The question I have is: will human evolution really continue? I think the evidence shows that human evolution has largely come to a halt. Nevertheless, to explore these questions about what a future utopia may be like – or more specifically, what version of humanity it will be populated by – we must first ask ourselves, what is evolution?

Darwin defined evolution, essentially, as descent with modification. I can rephrase this with an even simpler triad of words. Evolution is genetics plus time.

Each time DNA is copied, it is copied imperfectly, resulting in mutations. Given enough time, let’s say three and a half billion years, evolution is inevitable. It’s so simple it could almost be physics.

Evolution relies overwhelmingly on differences; in the genetic constitutions of different people, and in their ability to reproduce. Inherited differences in the ability to reproduce is what Darwin called natural selection.

That, too, is simple. If some individuals are more successful at finding a mate and raising offspring than others, and this is because of their genes, then these genes will spread at the expense of others. In time, the population will change and if it changes sufficiently, a new form of life will arise.

Evolution needs three components. First of all there’s variation, which comes from mutation. Second, natural selection, which comes from inherited differences between individuals and their ability to reproduce.

Finally, evolution is greatly promoted by isolation. Small, isolated populations, as Darwin noted when he visited the Galápagos Islands, evolve more readily or more obviously than others because their advantageous genes are not swamped and diluted by the movement of genes from outside the population.

My argument is that, for humans, all three components have more or less disappeared. We are undergoing a ‘grand averaging’, an event that is totally new in our evolutionary history.

Let’s consider the first component: variation via mutation. Perhaps the most cynical scientific experiment ever carried out began on 6 August 1945, when an atomic bomb was dropped on Hiroshima, Japan.

There was a strong belief that the children of those exposed to radiation would also be affected. Within a few months of the end of the war, a bunch of geneticists from the United States went to Hiroshima and Nagasaki.

What they did was compare the DNA of the kids whose parents had been exposed to a control group who had been out of the city at the time, to see if they could find an increased mutation rate.

By the year 2000, they had found 28 different mutations between the DNA of the parents and offspring. However, there was no significant difference between the number of mutations in the DNA of the group exposed to the bomb, and the control group.

As a species, Homo sapiens has been exposed to radiation for many years. Let’s consider the amount of radiation the average person is exposed to. We certainly get some from X-rays, some from gamma rays. And then there’s radon, a gas that leaks out of granite and is quite radioactive. There is little evidence that the bombs increased the human mutation rate at all.

However, there was an interesting aside to the data from the Hiroshima study. Of those 28 mutations noted, 25 had taken place on the father’s side rather than the mother’s.

By far the most dangerous of all mutagens, we now know, are men. Every time a cell divides there is a chance of an error being introduced into the DNA, and the more cell divisions there are, the higher that chance will be. The number of cell divisions between the egg that made a woman and the egg that she passes on, whatever her age, is between 20 and 24.

For a 28-year-old father (today’s mean age of reproduction in the developed world) there are some 300 divisions. This is because sperm is constantly created. For a 51-year-old father, the sperms cells have been through about 2,000 divisions. So, not only are there more mutations in men than in women, but the rate is dependent on age.

Despite this, we see little evidence to say that human evolution is going to speed up because there are more mutations coming into the process. Nowadays, people start their families late, but stop early; changing demographics show that there are fewer older fathers today than in the past. So, if anything, the rate of mutation is going to slow down.

Let’s turn to the second issue: natural selection. People often think of natural selection as something almost magical. But it isn’t. It’s extraordinarily simple. I first witnessed natural selection taking place in a soap factory in Liverpool in the 1960s, where I worked after leaving school.

Detergent was made then as it is made now: by forcing boiling hot chemicals at great pressure through a nozzle. As the mixture zooms out, the pressure drops, and it breaks into a vapour that is sucked away and a powder which is then sold as detergent.

The nozzles were a damn nuisance. They were inefficient, kept blocking and made detergent grains of different sizes.

Unilever and various other companies hired mathematicians and physicists in an attempt to improve the situation. But they didn’t do very well; it turns out that the physics and maths of the transition from liquid to powder is quite difficult to understand.

So, almost in despair, they turned to the lowly biologists and asked if they had anything to add. What the biologists did was to apply Darwinian natural selection.

They made 10 copies of the nozzles, with slight changes absolutely at random. Some nozzles were longer, some shorter, some had a bigger or smaller hole, maybe a few grooves on the inside. But one of them improved a very small amount on the original, perhaps by just one or two per cent.

Based on the improved nozzle, they made another 10 slightly different copies, and repeated the process. After only 45 generations – which would be an utterly trivial instant in evolutionary time – they had a nozzle that worked many times better than the original. This was without any forethought of any kind, only by a simple application of evolutionary mechanisms.

So, do we see evidence of this kind of natural selection in humans? One of the strange things about us as a species is that we’re spread very widely, and we’re also quite variable from place to place.

Consider skin colour. The distribution of skin colour very much fits with the amount of sunlight, and it’s related to the ability and the need to make vitamin D.

Through evolution, the main gene for black skin entirely differentiated between Africa and Europe: almost 100% of one kind in Africa, almost 100% of another kind in Europe. This gene turns on the production of vitamin D (ergocalciferol).

If a baby has a shortage of vitamin D, it gets rickets, but you can avoid rickets by having a diet high in vitamin D, or your body can make it via the action of ultraviolet light from the Sun.

The differences in the level of this effect can be quite striking. If you take a fair-coloured Scandinavian baby with just its face exposed and you put it in the midday Sun for an hour, it will make enough vitamin D to stay healthy.

However, if you do the same with an African baby with black skin, he or she couldn’t match that level of vitamin D production in a whole day. So individuals with relatively light-coloured skin were favoured as we moved out of Africa. Natural selection works on differences.

Now, consider the proportion of life and death of English babies from 1600 to 2000. In Shakespeare’s time, two out of three babies died before reaching the age of 21. In Darwin’s day, just half of them were dead. Today, that’s down to just one per cent. That’s a great achievement in the developed world, but for evolution it means that there are no differences in mortality to the age of reproduction, and therefore no raw material for natural selection.

The second part of the natural selection equation is to find a mate and reproduce. Women are limited by the simple facts of biology, but men can potentially have large numbers of children. And some men, indeed, do.

Consider Osama Bin Laden, who comes from a very powerful family and whose father, Muhammad Bin Laden, had 22 wives and 53 children. Last time we counted, Osama himself had five wives and 22 children. So we see massive reproductive success by those two men. However, the logical conclusion is that if some man has 22 wives, 21 other men have no wives at all.

This happened in Ireland’s history too. When we map the Y-chromosomes of Irish men, it turns out that there is a great splash of one particular type near Donegal. And from the history it’s clear that those Y-chromosomes descended from the high kings of Ireland.

One of them, for example, who died in 1423, had 18 sons, and descended from a warlord called Niall of the Nine Hostages, who also had many children. If any of the differences in reproductive success between these men and men with fewer offspring were associated with their genes, then that was raw material for natural selection.

Yet those differences have largely gone away. Today there is only a small difference in the number of children that different people have, irrespective of their genes. We know that the mean fertility in Europe has decreased because childhood mortality has decreased, but the interesting question is not about the average of the birth rate but the variance in it.

Between 1880 and 2000, the huge variation in the average number of children born per couple between countries diminished. So, once again, there’s no evidence that evolution has anything left to work with.

You can put the variance in rates of survival and the variance in rates of selection together and come up with a statistic that’s called the ‘opportunity for natural selection’. That’s been done in various places, most of all in India.

India is like a microcosm of world history in a way, because there is a large middle class whose lifestyles are very similar to the lifestyles we enjoy in developed countries. But, there is also an enormous peasant class similar to those of historic Europe who, for hundreds of years, lived off the land.

Furthermore, there are still a few tribal peoples who are living not exactly as hunter gatherers, but with a life much like that very early in the history of agriculture. The Indian Government keeps good records on its own patterns of birth and death so we can work out the opportunity for selection.

The records show that the variation in the number of survivors and the number of children has gone down by more than 90% in the middle class compared to the tribal peoples. And that’s a microcosm of the global pattern.

So let me turn to the final aspect of evolution, the role of isolation. One thing that’s very clear about humans is that there are a lot of us about. If you put us on a graph of abundance in relation to body size, we’re way off the line. There are many more mice in the world than elephants, but if you plot these two species of mammal, they sit on a straight line. Not us – we are 10,000 times as common as we ought to be.

On a typical day, you will see more people than the average hunter gatherer would have in a lifetime (and the hunter-gatherer lifestyle makes up more than 99% of human history). The natural human population isn’t 6.7 billion, it’s about half a million. Historically, we were very rare – and if you are rare and you cover a large area, then of course you live in small, isolated groups.

Finland is a great example for demonstrating the effects of isolation. The Finns, who have an effective genetics service, have 33 unique genetic diseases, called recessives (for which two copies of a disease gene are needed before the condition manifests itself).

These 33 diseases are found nowhere else. The Finns have incredibly good family records, too. If you look at the patterns of family history, it transpires that certain diseases are most prevalent – though still rare – in those small patches where there was a lot of inbreeding.

As a general rule, populations that are geographically isolated tend to become inbred. They tend to manifest their own set of genetic diseases but also their own genetic identity in general.

But today, people are on the move; no man or woman is an island any longer. Instead, the world is becoming a single genetic continent, and we can put figures on that. For example, how far apart were the birth places of you and your partner, if you have one?

How far apart were those of your parents, and of your mother’s mother and your mother’s father? Certainly, for most Europeans, in almost every case, that figure will get smaller and smaller as you go further back in time.

But now the world’s populations are mixing at an extraordinary rate. In Britain, one marriage in 50 or so is between members of different ethnic groups. We are mixing into a global mass; the future is brown.

There are several ‘genetic ancestry testing companies’ I have very mixed feelings about, to whom you send off some saliva for them to analyse your DNA and tell you that you are going to die on 17 September 2042 of a heart attack. Well, they’ll tell you that, but it’s not necessarily true.

They make most of their money out of people trying to find their ancestry. One particular group is very interested: African Americans, who feel that their history has been stolen from them. There is a great deal of interest among African Americans in finding out where exactly their genes come from.

The data that gets sent back will often tell them that parts of their chromosomes are African in origin, other parts are European and some parts are Asian, probably from Native Americans.

What we find is, even more today than in 1950, that our genes are mixing and averaging out. More and more people are subject to this averaging process. And for humans, this averaging means an end to evolution.

To sum up, when we look at the three factors needed for evolution – variation between people, selection pressure via differences in death rates, numbers of offspring and geographic isolation – we see that, for the time being, they have largely disappeared. For humans, evolution has lost its power.

So, if you’re worried what utopia might be like, you don’t need to anymore because you are living in it right now.

Steve Jones is a geneticist at University College London and author of many books, including Almost like a Whale.

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