Thanks for providing this service! I've been wondering: How do traits which do not manifest until old age get selected for? In other words, osteoporosis, fading hearing and eyesight, dementia and many other conditions do not show up in an individual until AFTER reproductive age. Therefore, these traits should have no bearing on "fitness" and thus no role in natural selection, yet they are not present equally in all individuals. So how do they get selected? Or do they simply NOT get selected, and their distribution is truly random?

Thanks in advance.

Joe D'Agostino

Hi Joe,

That's a fantastic question which has been thought about a lot by evolutionary biologists. Although there are some differences of opinion, all the conditions you mention generally happen late in life and are part of what we often call aging or "senescence". The basic point is that is senescense is bad for fitness, why doesn't selection get rid of it?

It's not true to say that they have no bearing on fitness at all, but it is fair to say that because they happen late selection is less effective at minimising their impact - precisely for the reasons you give. This means that for example, genetic mutations causing diseases late in life will they don't get selected out of a population very effectively. Also
some mutations may have a good effect early in life (e.g. make you more fertile) but a bad effect later. Beacuse selection will work more effectively early in life those kind of genetic effects may be kept in the population. There's quite a bit of evidence for these kind of mechanisms now.

So the bottom line is that these conditions aren't selected for, it's that selection against them is not very effective.       


Last edited by Alistair Wilson (2nd Mar 2007 10:50:00)