Thesis #35 – The ages at which the forces of natural selection plateau depend on the last ages of reproduction and survival in the evolutionary history of a population, allowing experimental evolution of the cessation of aging by deliberately changing those last ages in laboratory populations.

Earlier in the 55, it was pointed out that it proved relatively easy to manipulate the evolution of patterns of aging by the simple expedient of changing the first age of reproduction in laboratory populations over a number of generations.  This was the means by which the creation of Drosophila populations with slowed aging became a routine experiment of aging research.

My intuition in the 1990s was that changing the last age of reproduction in a laboratory population would change the age at the aging phase comes to an end, over the course of evolution.  So I persuaded Larry Mueller to simulate the evolution of populations which were subjected to such a change in their conditions of evolution.  As I had hoped, my intuition was correct:  changing the last age of reproduction or survival in a population’s life cycle eventually leads to the evolution of corresponding changes to the timing of the cessation of its aging.  Specifically, populations with a later last age of reproduction evolve a later cessation of aging.

As it happened, my laboratory had a spectrum of Drosophila populations that had undergone evolutionary shifts in their last ages of reproduction over hundreds of generations, so it was relatively easy to check whether or not the ages at which their aging ended had shifted in the expected manner.  Briefly, what we actually found in our populations qualitatively corresponded to what Larry Mueller’s numerical simulations had predicted:  fruit fly populations which we cultured for hundreds of generations with an early cessation of reproduction during adulthood had an earlier cessation of aging, relative to populations with a later cessation of reproduction.

What this showed was that evolution readily tunes the cessation of aging according to the forces of natural selection, just as it tunes the rate of aging.  There is nothing difficult, for evolution, about changing when the aging of a species stops.

So we went a step further.  We tried changing the age at which reproduction ceased over just a small number of generations, about twenty.  To our surprise, the age at which aging stopped then evolved even faster than the rate of aging evolved.  It is as if there is much less evolutionary friction affecting the cessation of aging than the rate of aging.  In principle, this implied that changing the point at which our aging stops might be an easier project than changing our rate of aging.

However, when we wrote up this result for publication about ten years ago, we still didn’t realize how easy it would be change the point in an animal’s life at which aging stops.  But more on that later.