Fertility is declining in younger women, but could be increasing among the older, thanks to changing marriage patterns, finds a new study.
Unlike in the past, women these days do not start childbearing until an older age as marriage is often delayed, and casual or short-term relationships and divorce are more common.
As a result, the natural selection maintaining young-age fertility might weaken and the relative strength of natural selection on old-age fertility could increase. This phenomenon could potentially lead to improvements in old-age fertility over many generations, theorize researchers at the University of Sheffield.
They studied Finnish church records from the 18th and 19th centuries, a time during which almost everyone married and divorce was forbidden, to trace the survival and marriage histories of 1,591 women.
The researchers found that women aged 30-35 were the most likely to be married. Those that married wealthy husbands were married at a younger age but to relatively older men, thereby gaining the family size-benefits of wealth but also an increased risk of widowhood. This high chance of widowhood, coupled with low re-marriage prospects for older widows with children, limited the percentage of women in the population with the opportunity to reproduce at older ages.
Things have changed drastically since. "In many animals, including humans, the ability of females to reproduce depends not only on their survival to each age but also on being pair-bonded to a mate. Exposure of the genetic variation underlying fecundity to natural selection should therefore depend on the proportion of females both alive and pair-bonded. In spite of this, female "marital" status is seldom considered to impact the strength of selection on age-specific fecundity," researchers Duncan Gillespie, Dr Virpi Lummaa and Dr Andrew Russell, from the University's Department of Animal and Plant Sciences, said in their article "Pair-bonding modifies the age-specific intensities of natural selection on human female fecundity."
They concluded, "Our results suggest that the decline in selection intensity after age 30 years is a factor in the evolutionary maintenance of female reproductive senescence and menopause."
The paper is carried in Pubmed, a free digital archive of biomedical and life sciences journal literature.
Duncan Gillespie said: "In today's society, family-building appears to be increasingly postponed to older ages, when relatively few women in our evolutionary past would have had the opportunity to reproduce. As a result, this could lead to future evolutionary improvements in old-age female fertility.
"Childbearing within a relationship is still the norm in modern society, but at ages where fewer women have the chance to reproduce, we should expect the evolution of lower fertility."