Reversing Cart and Horse in Reduced Late Life Selection Pressure and Evolution of Aging

The present consensus on the evolution of aging is that selection pressure is lower in late life, inevitably, because there is always greater advantage to early life reproduction in an environment of hazards and predation. Thus species will evolve bodily systems that work well in early life, but fail over time, because mutations that provide early life benefit will be selected even when they cause later harm and decline. That said, researchers here produce models to suggest that the arrow of causation runs the other way, that evolution will simply produce lowered late-life selection regardless. As in all evolutionary modeling, this might be taken with a grain of salt for now, and treated as an interesting idea for discussion only; the field generates a great deal of hypotheses based on modeling and little else.

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.

In our model, ageing is evolutionarily inevitable in a dynamical sense irrespective of the genetics of fecundity and survival. Selective forces may at times be stronger in late life than in earlier life. But, as we show, this property pertains to either a transient or an unstable state, which is eventually abandoned. An ever-declining force with age is not an intrinsic property of selection and the one driver behind the evolution of ageing, as the classic theory implicitly assumes. Instead, a persistent, age-related weakening of selective forces is itself a result of evolution. Our model may then be viewed as a generalization of the classic theory where an implicit assumption of the latter is turned into a prediction.


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