Looking into how paternal age affects life history evolution, I took our data on survival curves of Herero males and calculated the strength of selection, that is the derivative of fitness with respect to survival, as a function of mean paternal age. The computation is just equation 16.3 on page 116 of Alan Rogers’ textbook that I mentioned in a previous post.
The graph shows the strength of selection on mortality by age in two model populations. In the young fathers version, the birth rate (to males) is uniform from age 20 to 30, In the old fathers model the birth rate is uniform from age 30 to 50.
No surprises here save one. While selection for survival should extend male lifespans by 10 to 20 years in the case of old fathers, selection for survival before the age of reproduction is much weaker in the the case of older fathers. A prediction is that adolescent and young male death rates should be higher in old father societies because selection is weaker. I never realized that.
Hamilton’s theory does not describe human life history very well, as Rogers shows in his Figure 16.1 and discusses in the text. Human female fertility ceases long before the theory predicts that it should and humans live much longer. The reconciliation certainly has to do with kin selection or indirect selection. For example Kris Hawkes pushes the “grandmother hypothesis” according to which females cease reproduction and instead work for their daughters’ children. If she is right this grandmother effect selected for the prolonged human lifespan, and the long lifespan of males is a side-effect of selection for long life in females.
Hamilton’s theory also fails to predict the mortality peak in infancy and early childhood. Here again indirect selection ought to be brought in. A frail infant with no prospects “ought” to die early since the death frees up the mother for the next child, an important effect since the human female reproductive span is short and every year counts.
Rogers has worked much of this out, incorporating kin selection into life history theory, in a paper several years ago. The paper is, to put it mildly, somewhat difficult to read. Good luck.