Dr Ian Hastings is a scientist at the Liverpool School of Tropical Medicine. He believes that 'reproductive compensation' is having a significant effect on the gene pool.

Could you explain reproductive compensation?
Think back to maybe 100 years ago where families tended to be very large and a mother would reproduce as quickly as she could. Now, imagine she had 13 children, that’s as many as she could physically produce, and one of them died of genetic disease, then that dead child has gone and taken his or her genes with them. There’s no capacity within the mother to produce another child. Contrast that with the modern situation, where people plan to have maybe 2 or 3 children – if one of those children die of a genetic disease, it’s well within the capacity of the mother to produce another child to compensate for that dead child, and the tendency is for that replacement child to be carrying the gene. In fact, if it’s a sex linked recessive disease like muscular dystrophy there’s a one in three chance that the replacement child will carry a copy of the mutated gene, and as a consequence of this the frequency of the mutated gene increases in the next generation.

What does this increase of mutated genes mean for evolution?
What are the consequences of that for long term human evolution? I think that depends on how you view the human gene pool. There is a tendency to view humans as being at the pinnacle of evolution, or maybe even races as being the pinnacle of evolution, and that any dilution of that gene pool would be catastrophic, we’d fall off a pedestal in other words. But evolutionary geneticists are far more relaxed about it, they look at physical aspects of humans and say they’re not particularly fast, they’re not particularly strong, they survive in the modern world because of their culture, and it’s culture rather than genetics that’s the important thing. We also now know that most people in the population carry 2 or 3 lethal recessive mutations, so it’s not particularly good gene pool anyway and if it degrades a little bit, well, probably that’s not going to have huge consequences for human evolution, at least not over our life spans and the life spans of our great-great-great-grandchildren.

We can quantify this increase of frequency of genes in the next generation mathematically, if we assume that historically there was no reproductive compensation and that now we do compensate. So, for recessive genes the frequency will rise by about 0.5% per generation, but for sex-linked lethal genes the frequency change will be very large, it may increase by 15-20% in the first generations, and over the first few generations will increase maybe 30-50%. It will then stabilise at a new equilibrium value.

How is modern medicine affecting evolution?
So mutations inevitably occur and come into the human population, the question is what happens to them thereafter. Now, in traditional evolutionary genetics of animals and plants the only way they’re eliminated is if the individual dies or fails to reproduce. Neither option is particularly attractive to human beings, so we’ve developed medical techniques to try and cope with these mutations. There are two real ways of trying to deal with them: the first is gene therapy, they may well try in the future to reverse these original mutations; and the other way to do it is just to treat the symptoms of the disease. Haemophilia is a good example of something which was lethal up to fairly recently and is now fairly easily treated by modern medicine.

Do genetic diseases or medicine have the upper hand?
Historically infectious diseases have been an important selective pressure on human populations. Biologically they always had the upper hand – they had big population sizes and they had a very rapid population growth, and they always kind of stayed one step ahead of humans. Now the situation has reversed, rather than genetic evolution we’re in the situation of cultural evolution, and medicine now can evolve more quickly than the mutations can. So, at least long term, we are optimistic that we should stay ahead of diseases as a selective element.

How will reproductive technology affect the rate of mutation?
Sex-linked recessive diseases such as muscular dystrophy (DMD) only affect boys. So we can anticipate the situation whereby families can choose the sex of their offspring, and if the mother carries a copy of the DMD gene, they may well decide just to have girls. Now, if that does occur because the girls tend to be carriers of DMD, then the frequency of the gene will rise quite rapidly within the population.

Does all this talk of genetic change need to be put into some sort of context?
Yes, the critical point to note is that evolutionary geneticists tend to think in the timescale of generations - a human generation will be maybe 25 years. Now, our timescale is usually hundreds or thousands of generations, which if you convert that to real time in humans is a vast amount of time. Even ten human generations, which is very quick on an evolutionary timescale, would be 250 years. So if you think back to 250 years ago and the state of human society then, you realise how slowly genetic processes occur compared to cultural evolution.

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Content last updated: 17/07/2006

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