The study – by researchers from the University of Turku (Finland), University of Exeter (UK), University of Sheffield (UK) and Stanford University (US) – explains for the first time why the relationship women had with their daughter-in-laws could have played a key role.
The data showed that a grandmother having a baby later in life, and at the same time as her daughter-in-law, resulted in the newborns of each being 50 per cent less likely to survive to adulthood.
The analysis helps to solve one of nature’s great mysteries: why female humans, unlike most other animals, stop reproducing so early in life.
It also adds weight to the theory that the menopause evolved to allow women to focus on their grandchildren. Traditionally, this role included providing food for the family and protecting young children from accidents and disease.
Read more at ScienceBlog.
If a grandmother (GM) and her daughter concurrently produce babies, the daughter will be sharing 50% of the GM’s genes and the grandchild will share 25% of the genes of the GM. If the GM and her daughter-in-law concurrently produce babies, the daughter-in-law will (usually) be sharing 0% of the GM’s genes and the grandchild will again share 25% of the genes of the GM.
Therefore one would expect the GM to give equal priority to the survival of the grandchild in both instances. However, from a purely evolutionary point-of-view, one would expect the GM to prioritize the survival of her daughter far higher than the survival of her daughter-in-law.
If there is conflict between the GM and the daughter-in-law for resources, I can’t see that there is Darwinian selection pressure to suppress the fertility of the GM. If the daughter-in-law doesn’t survive, the GM’s prospects for passing on her genes via her son may decrease, but only marginally.
The results and conclusion of this study would therefore appear surprising. Have I missed something here?