The selection force weakens with age because ageing evolves and not vice versa

A decline of selection with age is generally seen as the reason that ageing evolves. But selection can also increase with age. What happens then? This work shows that ageing nevertheless evolves, and so does the decline of selection with age. According to the classic theory of life history evolution, ageing evolves because selection on traits necessarily weakens throughout reproductive life. But this inexorable decline of the selection force with adult age was shown to crucially depend on specific assumptions that are not necessarily fulfilled. Whether ageing still evolves upon their relaxation remains an open problem. Here, we propose a fully dynamical model of life history evolution that does not presuppose any specific pattern the force of selection should follow. The model shows: (i) ageing can stably evolve, but negative ageing cannot; (ii) when ageing is a stable equilibrium, the associated selection force decreases with reproductive age; (iii) non-decreasing selection is either a transient or an unstable phenomenon. Thus, we generalize the classic theory of the evolution of ageing while overturning its logic: the decline of selection with age evolves dynamically, and is not an implicit consequence of certain assumptions.

Automated summary

A decline of selection with age is commonly believed to be the reason for the evolution of ageing, but selection can also increase with age. This study demonstrates that ageing still evolves and the decline of selection with age also evolves. Therefore, the changes in selection force with age are a dynamic result rather than an implicit consequence of specific assumptions.