Saturday, September 22, 2007

Old People and a Silly Press Release

Read an interesting piece of research by Stanford University researchers on why humans live well beyond their reproductive years. I first read it in Science Daily, but here is the original press release from Stanford University. Organisms that cannot reproduce are irrelevant evolutionarily. So natural selection should ideally design the human life cycle to cease once our reproductive years are over. But women live well beyond menopause into their 70's and 80's. One explanation is that women need to live long enough to raise kids in to their independent years. But this could raise longevity to the 60's at most. Another explanation is the "grandmother hypothesis" which suggests that old women by helping raise grandchildren successfully, will pass genes for longevity through their grand kids, so over time increasing longevity beyond menopause. Both these theories are plausible, and the Stanford researchers have added another.

They look at male reproductive biology. Unlike women, men do not have their fertility suddenly cut off at menopause. Rather it declines gradually. But if a man's life partner hits menopause, then shenanigans aside, the man also becomes evolutionarily irrelevant and should face death. However, since most men marry younger women, this irrelevance is postponed to later years, and if older men take very young wives, then it may be postponed almost indefinitely. So, because over human history, very old men have taken very young wives, genes that cause harmful mutations later in life have been steadily winnowed out by natural selection and genes for longevity have accumulated. Because over evolutionary time, a gene spends roughly half its time in women, their longevity has increased as well.

All of this very interesting, except starting from the Stanford press release, all news organizations describe this as an example of evolution as good for the species!

"It turns out that older men chasing younger women contributes to human longevity and the survival of the species" - states the Stanford press release


"But the fatherhood of a small number of older men is enough to postpone the date with death because natural selection fights life-shortening mutations until the species is finished reproducing". (this off course should be "until the individual has finished reproducing", but it shows how deeply embedded is our thinking that species is what matters in evolution).

The emphasis on the term species is mine. This is an old fault in our thinking to visualize individuals as striving for the benefit of the species. Humans and all organisms behave in ways that increase their own chances of survival and reproduction. That behavior may have an effect that has some benefits for the larger group, but that does not mean individuals are behaving for the benefit of the larger group. What would our reproductive behavior be if we were acting for the benefit of the species and not for ourselves? For one, evolution would have led to behavior that strictly imposed a limit on the number of offspring, an evolutionarily imposed population control. That does not seem to be the case. Secondly, since the quality of gametes decreases with increasing age, we should have evolved behavior that imposed a limit on the age that we reproduce. But men especially seem to retain fertility until a very old age.

Surprising that a University as renowned as Stanford will make such mistakes, but it shows how little control scientists have over even their own organization's press offices. The good of the species theme appeared in Times of India too. Besides that mistake, Times of India missed out on something else. One of the lead researchers from the Stanford group involved in this work is an Indian, Shripad Tuljapurkar. Our media missed out on a big opportunity to do some major chest beating :)


  1. Ha ha, well spotted! The group selection statement is something which caught my attention as well.

    While I agree that the stamenent is very misleading, one can indirectly justify the statement, using the selfish gene theory that Dawkins discussed in his book.

    IMO, ultimately genes are a set of chemical molecules which have the property of acting as replicators, and the survival of a gene depends on whether it is able to replicate. Now, if there is a mutation in a gene such that phenotype in the "machine" which carries the gene is able to act altruistically to other machines which carry the same gene, then it is beneficial to the survival of the individual gene. So, altruism, and survival of the species of a whole (where the members of the species carry similar genes), can be explained in terms of the survival (ie, replication) of individual genes.

  2. sure. my objection is not that behaviors or traits do not benefit the group, but that they evolved because they are of benefit to the group. I agree with your "selfish gene" statement. In many cases the "interests" of the genes coincide with the interests of the group.