Apparently he said the following:
I think that we’ve stopped evolving. Because if natural selection, as proposed by Darwin, is the main mechanism of evolution – there may be other things, but it does look as though that’s the case – then we’ve stopped natural selection.
We stopped natural selection as soon as we started being able to rear 95–99 per cent of our babies that are born. We are the only species to have put a halt to natural selection, of its own free will, as it were.
Now, I idolize the man, but the point he’s making is just wrong. I’ve also had a brief Twitter conversation on this, and promised to do some homework, so here is the result.
First, why is Attenborough wrong? In short, because you can’t stop natural selection, you can only divert it. Natural selection will always be active, and if the main selective forces may disappear for any given reason, other selective forces simply become more relevant. Sir David is right in pointing out that the moment we became able to “rear 95–99 per cent of our babies” we changed things dramatically. With civilisation, we decreased the selective pressures that favoured people able to stay away from dangerous predators, hunt effectively and so on. With the recent advances of medicine, we also stopped (most) selection on our ability to survive or escape deadly infections. That’s true. However, if a genetically inheritable trait gives a higher probability to produce a larger-than-average offspring, this would still increase its frequency over the generations. The reason is already in my definition above so I don’t think it needs an explanation; in fact, it is so inevitable that it doesn’t even need to be empirically demonstrated (admitting that this would be possible, when dealing with humans).
I will substantiate this claim with two examples (also because I mentioned one on Twitter):
Example 1, taken from The Journal, quoting Dr Ian Rickard (evolutionary anthropologist at Durham University):
Consider gene variant A that predisposed someone to put on fat more than the alternative variant B.
Before the widespread availability of high energy foods in Europe, gene variant A might be more common than B because it helped people retain their energy balance under conditions of food shortage, allowing them to survive and eventually reproduce.
However, nowadays A would be more likely to lead to obesity and diabetes and premature death, perhaps before the person with the gene had children.
Having the B variant would not be so disadvantageous when there’s lots of high-energy food available.
This means that B would be now be relatively more advantageous than A. As a result, natural selection against A would decrease, and selection for B would increase. If the selection pressure were consistent over time, this would cause B to become more common, and A to become less common
Seems an obvious and inevitable prediction, given what we know about inheritance. You don’t even need to accept the full theory of evolution, you just need to accept that inheritable traits may or may not predispose people to put on fat.
Example 2 (my own) can be seen as a variation of the first one.
Say that you have an inheritable trait that makes you less rebellious during the adolescence. If you have this trait, and happen to be raised in a strict Catholic family that fully embraces the Church opposition on birth control, you’ll have more chances of retaining this belief, and will consequently have higher chances of producing many children. These will have a higher than average chance of being less rebellious, and will therefore reinforce the trend. The selective pressure here is created by the different attitudes of different cultural groups, so this example is also a neat explanation of the basic idea behind what can be described as genetic and cultural co-evolution (the ideas will tend to spread along with the genes).
One may think that the question becomes: does such a trait exist? Well, it’s irrelevant, because traits that favour different degrees of retention of learned beliefs certainly exist, and if they don’t, they will be favoured as and when they will emerge. As it happens, in the current (Western) world, when a “stick to your family’s beliefs” trait appears in a cultural group that encourages large families (doesn’t need to be a religion, but in the world I know they usually are), such a trait will spread over the generations, and because it will be associated with a particular culture, the culture itself will be favoured as well. This is not an opinion, it is again the direct consequence of verifiable evidence. This is why I’ve tweeted “You can’t stop nat-sel. It’s still active, promoting new things, such as religiosity, as it favours large families”.