One evolutionary quandary that has plagued biologists is the existence of a menopause in women—the time after which a woman loses her ability to reproduce. Why would females of a species stop reproducing? What is the evolutionary advantage to this? Wouldn’t it be better for a woman if she could keep producing offspring and thus propagating her genes for as long as possible? In fact, menopause is relatively uncommon in animals, though it has not been widely studied.
Grandmothering Effect and Other Hypothesis
One hypothesis that has been proposed to explain this is the aging effect. As women age, they are more likely to develop mutations that they pass on to their children. Stopping their ability to reproduce could help protect the species from defective mutations. Another hypothesis is the ’Grandmother effect’. If older women use their resources to help bring up existing off-spring instead of creating new ones, the additional resources given to a child increases its fitness.
A research team has used data from a pre-industrial Finnish population to test these hypotheses. Since the 17th century, the Lutheran church has collected a register of all births, deaths and marriages in Finland. The researchers had access to three generations of 5 Finnish populations.
Mothers-in-Law Compete with Daughters-in-law, But Not With Their Daughters
Their most interesting result was that when a mother-in-law and a daughter-in-law had children within two years of each other, their off-springs had significantly smaller lifespan by up to 66%. This suggests that competition between in-laws have led to menopause evolving as an adaptation. On the other hand, when a mother-daughter pair had children within two years of each other, no reduction in children’s ages were seen.
A woman shares no genes with her daughter-in-law, and her grandchild only shares 1/4th of her DNA as opposed to her child (which has ½ of her DNA). Thus, there is no co-operation between the two during child-rearing leading to reduced fitness of all the off-spring. It is interesting to note the contrast when mother-daughter pairs bear children simultaneously and show no competition. The authors suggest that in-laws fought over resources for their children instead of co-operating as mothers and daughters might do.
Modeling this data, the authors of this research have proposed a combination of factors that lead to the evolution of menopause. The first is the grandmother effect, and the second is the avoidance of reproductive conflict between mothers-in-law and daughters-in-law.
You can read about this research here.