Sunday 8 January 2017
A common misunderstanding of natural selection
My attention was drawn today to an article in the Atlantic, entitled ‘Why Do Humans Still Have a Gene That Increases the Risk of Alzheimer’s?’ It noted that there are variants of the apoliprotein gene that are associated with an 8- to 12-fold increased risk of the disease. It continued:
“It doesn’t make sense,” says Ben Trumble, from Arizona State University. “You’d have thought that natural selection would have weeded out ApoE4 a long time ago. The fact that we have it at all is a little bizarre.”
The article goes on to discuss research suggesting there might be some compensating advantage to the Alzheimer risk gene variants in terms of protection from brain parasites.
That is as may be – I haven’t studied the research findings – but I do take issue with the claim that the persistence of the risk variants in humans is ‘a little bizarre’.
The quote indicates a common misunderstanding of how natural selection works. In evolution, what matters is whether an individual leaves surviving offspring. If you don’t have any descendants, then gene variants that are specific to you will inevitably disappear from the population. Alzheimer’s is an unpleasant condition that impairs ability to function independently, but the onset is typically long after child-bearing years are over. If a disease doesn’t affect the likelihood that you have surviving children, then it is irrelevant as far as natural selection is concerned. As Max Coltheart replied when I tweeted about this: “evolution doesn't care about the cost of living in an aged-care facility”.