Sunday, 8 January 2017

A common misunderstanding of natural selection



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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”.

5 comments:

  1. And given the prevalence of back problems in humans, he says flexing carefully, one wonders why evolution ever 'allowed' us to walk upright. Surely evolution should have provided periscope eyes or a third pair of limbs. Or both!

    OT but fun; https://wtfevolution.tumblr.com/

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  3. It's a good point and mostly true, but I feel it's worth adding that natural selection can sometimes act in more subtle ways, promoting genes that don't directly aid the survival and procreation through the individual, but through the dynamics of a herd/pack/tribe. Altruism, for example, is a trait that doesn't confer a particular benefit to individuals that have it, but an "altruism gene" may still be promoted by natural selection because tribes in which it occurs have more surviving offspring on the whole. Similarly, a lifespan beyond an organism's reproductive years can still have an evolutionary benefit if the older individuals can help provide for the tribe. So when it comes to traits or weaknesses that don't manifest until old age, the selection pressure is certainly weaker, but not absent.

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    1. Has anyone done serious estimates of the impact of this in humans? I assume it would depend on overall life expectancy in early humans.

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    2. I don't know about that - I'm not an expert. My main familiarity with particular subject this is through the so-called "grandmother hypothesis" (https://en.wikipedia.org/wiki/Grandmother_hypothesis), which is a theory that attempts to explain the existence of the menopause in human females (which I believe is quite unique to our species). It proposes that menopause isn't merely a loss of function, but rather evolved specifically so that women could redirect their energy from producing and caring for their own children, to caring for the next generation. The idea being that after a certain age, this becomes a more efficient use of energy to promote the survival of one's genes.

      I agree these effects would depend on how long individuals could actually be expected to live after their reproductive age, but then again that itself is something that could presumably change with evolution. E.g. it might be advantageous for certain species to keep both males and females around as long as they can contribute to the group, and so genes promoting longevity might get selected for. And I guess you'd expect this to happen more for humans than, say, wolves, as we can be useful to others in ways that aren't so physically demanding (and thus would decline more with age), such as teaching, cooking, etc.

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