Dr Adam Eyre-Walker is an evolutionary biologist at the University of Sussex. Dr Eyre-Walker recently published a paper investigating the rise of harmful mutations in the human population. He believes that human evolution is still happening, although natural selection is relaxed in the developed world.

What is a ‘harmful mutation’ and what results have you got from your research into them?
A harmful mutation is a mutation which reduces the probability of producing offspring, so that it either reduces the probability of surviving to reproductive age or the probability of actually being fertile. Most mutations are thought to be spontaneous, each generation your DNA has to be replicated several times and it’s in during that process most of the mutations are thought to occur.

We’ve recently estimated the rate of these harmful mutations since we split from chimpanzees about 6 million years ago and we came up with an estimate of about 2 harmful mutations occurring per genome per generation, which is the highest that’s ever been estimated in any organism so far and that’s probably and underestimate. It means that we are all carrying maybe up to 1000 harmful mutations.

Hasn’t changing cultural attitude also affected natural selection?
Yes, in the past some couples would have many, many children – you could call those ‘ultra-fertile’ couples - and some would have none. Now, however, variation in family size is quite small, most couples now have 2 or 3 children because it’s no longer behaviourally the thing to do to have many. Therefore the ultra-fertile couples, or the potentially ultra fertile couples, have essentially vanished, they no longer have an advantage over the less fertile couples. And therefore selection has been relaxed on fertility.

Why do genetic diseases still exist?
There are many genetic diseases around largely because of recurrent mutation, so you’re just getting mutations always coming into the human population and those mutations will cause an individual to have lower fertility or something like that, but they may still have offspring, they may still have children who have that mutation. Furthermore, many of the genetic diseases we see in our population have late onset, so they affect the individual basically after their prime reproductive years and therefore natural selection is really not acting upon those mutations.

And finally there are the occasional examples where it may be the case that the mutation has some advantage in some environments, and the classic example of that is sickle cell anaemia, where the mutation makes you immune, or partially immune, to malaria but in some cases it will kill an individual. It’s also been hypothesised that diabetes might have been an ancient adaptation to starvation, so again that’s a genetic disease which in the past or in a different environment may have been advantageous but in a modern setting has become harmful to us.

How much do infectious diseases impact on evolution?
It’s clear infectious diseases have been a major force in evolution both in the past and in our current time. A dramatic example is the 1918 outbreak of influenza known as the Spanish influenza, which killed between 20 and 40 million people worldwide. And of course with the decline in efficiency of antibiotics many, many bacterial pathogens will again become very potent forces driving evolution, and individuals who are resistant to those are going to better than those who are not. So I believe that they will be a major force. You said mutations are increasing, and this is especially due to modern medicine.

Could we be facing mutational meltdown?
Mutational meltdown is the process by which as harmful mutations accumulate in a population, those harmful mutations, because they reduce things like fertility, can actually lead to a reduction in the population size. As soon as the population size has reduced, that actually increases the rate at which harmful mutations accumulate in the population. And of course as more accumulate the population size becomes depressed, that leads to the faster accumulation of harmful mutations and you can reach a critical point where those two processes basically snowball, you have positive feedback and eventually the population just becomes extinct.

Whether we are likely to go through that mutational meltdown I very much doubt it. It’s much more likely that what will happen is that we accumulate mutations through improved living conditions, modern medicine, and then if those sort of props are removed then we may find ourselves in a rather sorry state. But it’s always very important to remember that this is only true of the developed world. The developing world natural selection is much much more potent, selection is not relaxed anything like to the same extent as it is in the developed world.

Is it possible to argue human evolution has stopped?
I don’t think human evolution has stopped, for a number of reasons. First of all it’s alive and kicking in the developing world. Secondly, even within our own developed world there are still natural selection is working through some of these infectious diseases and is likely to continue to work. And finally, even if natural selection was completely removed evolution would continue because evolution is just the process of change, so if we relax natural selection then we would start to accumulate harmful mutations, or mutations which were harmful in the past, and that would lead to things like a slow erosion of fertility, a decrease in survival rates, and things like that.

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Content last updated: 17/07/2006

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