User:Samsara/TANS

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It has been traditional for much of evolutionary theory's history, and specifically in modern evolutionary study of life histories, to partition fitness into two components: survival (longevity) and reproduction (fecundity). This had great appeal initially, as it meant that if you wanted to determine the lifetime reproductive success (LRS) of an individual, you merely had to multiply those two quantities - longevity * fecundity = LRS, and biologists were interested in understanding the trade-off between the two: if you selected for survival, would you get lower fecundity or not, and could you show that the compromise made by nature resulted in maximum reproductive rates?

Ah, but M. Malthus! He was the one that started it all, got old Darwin the Younger thinking. And back in Malthus' day, it was about the rate of increase of a population. Well, we meet this theme again, because in evolution, what counts is not how many offspring you produce, but how quickly, as long as this speed can be maintained across generations. For much the same reasons, it is the geometric mean number of offspring that will be maximised by natural selection, not the arithmetic mean.

So a closer approximation of what natural selection maximises is not the product of longevity and fecundity, but the number of offspring weighted by the inverse of your age when you produced them. This, incidentally, explains ageing.