Saturday, May 10, 2008

Selection for genes in post-reproductive ages


I have to quibble with some text from Matt Ridley's otherwise awesome book Genome (1999). On page 201 he writes

Natural selection cannot weed out genes that damage the body in post-reproductive age, because there is no reproduction of the successful in old age.

The idea behind this is that natural selection works through gene propagation by those most able to reproduce. The phrase "survival of the fittest" (not Darwin's, by the way) is a bit misleading-- it doesn't really matter how well or how long you survive, only how much you reproduce. So if a person can no longer reproduce, then having great lungs in old age, or having crappy lungs in old age, it doesn't matter; it can't affect evolution.

Makes sense, doesn't it? The problem is that people past their reproductive ages often help raise and protect their descendants. That is, grandma and grandpa survive long enough to be able to effectively help out in raising their little grandchild Simon. Simon will out-compete Alexander, whose grandparents died early. Since Simon has his grandparents' longevity genes (genes that allow him to live into an older age), those genes will get propagated.

So goes my thinking, anyway. Note that I am not an evolutionary biologist.

Pictured is Simon, the Ocelot who out-competed Alexander's genetic ass!

REFERENCE

Ridley, M. (1999) Genome. The Autobiography of a Species in 23 Chapters. HarperCollins.

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