There is considerable debate over the origin and ongoing role of post-reproductive life span in evolution. If evolution is driven by reproductive success, why do the individuals of some species live on far past the capacity to reproduce? Our own species is an exceptional example of the type; we are very long-lived in comparison to other mammals, and have a long period of post-reproductive life. Why is this the case? You might look at the grandmother hypothesis for one possible answer to that question, thought it is far from being the only answer. In general one should expect anything involving biology to be intricate, complex, and composed of many separate mechanisms. In this paper researchers explore the concept that post-reproductive lifespans might serve as an aid to adaptation, for example. It is a good reminder that much of the debate over the evolution of aging is quite abstract and technical, based on the use of models and the limitations and bounds of effectiveness of those models:
Post-reproductive lifespan is a life history trait that is typical of some mammals, and is especially developed in humans who can live for decades after having given birth to their last offspring. To our knowledge, the role that survival beyond the last reproductive age may play in population genetics models of adaptive evolution has not been investigated yet. This is probably because models with ideally infinite census size are insensitive to the presence of post-reproductive individuals.
It is well known that post-reproductive lifespan reduces the population effective size, which is a parameter inversely proportional to the genetic change in a finite population due to random sampling of gametes through generations; i.e. drift. The classic model for populations without age structure suggests that a reduction in effective size goes to the detriment of fixation chances of a beneficial mutation. Hence, one would intuitively be led to infer that a more prolonged post-reproductive lifespan slows adaptive evolution in a population. In the present paper, we will show that this intuition is wrong and explain why it is so.
We compare two separate, stationary populations living in a constant environment that are equivalent except for the average time their respective members spend in the post-reproductive stage of life. Using a recently derived approximation, we show that fixation of a beneficial mutation is more likely in the population with greater post-reproductive longevity. This finding is surprising, as the population with more prolonged post-reproductive lifespan has smaller effective size and the classic population-genetic model would suggest that decreasing effective size reduces fixation chances of beneficial mutations. Yet, as we explain, in the age-structured case, when effective size gets smaller because of longer post-reproductive lifespan but census size is kept equal, a beneficial mutation has a higher likelihood to get fixed because it finds itself at higher initial frequency.