New evidence suggests that natural selection is leading women to have their first child at earlier ages. This is shown to be an inherited evolutionary change that is taking place despite the influence of social factors such as religion and education.
The findings, by a team of British, Australian and American scientists, show for the first time that both genetic and social factors, such as religion and education, are having a profound effect on the timing of human reproduction. These factors are influencing human evolution to a greater extent than at any time in the pre-history of humans.
Writing in the latest edition of the journal Evolution, the researchers reveal that women who reproduce earlier in their lives have higher Darwinian fitness. Fitness is the term that describes the ability of an organism to reproduce itself because it is well adapted to its environment.
They show that this trend is being passed on from generation to generation through natural selection -- the process in which successful genes pass on to successive generations through evolution. (In general terms, a generation represents 21 years.)
These findings are different from previous studies of pre-industrialized populations, in which the age at which women start their periods is the best predictor of the number of children they will eventually produce.
The differences in behavior are probably due to changes in society that occurred alongside industrialization, such as freely available contraception for women and better healthcare and nutrition for children.
Dr. Ian Owens of the Department of Biology and Biochemistry at Imperial College, an author on the paper, described the significance of their research:
"The most important finding is that changes in society, such as freely-available birth control for women and eradication of several important childhood diseases, which have taken place in the last 20 to 30 years will probably lead to genetic changes in humans through evolution.
"We can say this because the sorts of factors that we found that are associated with reproduction now aren't the same ones that people had found in pre-industrial populations."
The findings were made after an exhaustive three-year analysis of questionnaire data from 2,710 pairs of identical and non-identical twins held in the Australian Twin Registry (ATR).
After removing the influence of differences in religion, education and the effect of the baby boom, the researchers still found heritable genetic differences in three key "life history" traits. These are the age at which women started their periods; had their first baby and reached menopause.
Each of these traits was found to be genetically associated with fitness -- the ability of an organism to reproduce itself because it is well adapted to its environment.
Out of the three traits it is the age that women had their first baby which is most significantly associated with reproductive fitness. This means that genes influencing an early age of reproduction in the female population will become more common. In other words, women's genes will predispose them to start reproducing earlier.
Dr. Owens believes this work has identified a "genetic phenomenon" that is likely to have many implications in terms of how human behavior will change over the next few hundred years.
Evolutionary biologists have long agreed that the life history of modern humans has been molded by evolution due to selection that occurred in the past.
Hotly debated, however, is whether human life history traits are still under selection, and whether changes in human culture have led to new forms of selection.
Part of the twin data analysis aimed to discover the effect that social, psychological and historical factors had on the number and timing of children born to the 2,710 pairs of twins studied.
The researchers found many of the variations in the three traits were controlled by social factors such as religion and education. For example, Roman Catholic women had 20 per cent higher reproductive fitness than other religions. University educated women had 35 per cent lower fitness than those who left school as early as possible.
"I was staggered by the results we got," said Dr. Owens. "When we decided to control for these factors, I wasn't expecting anything to come out of it. I thought, 'let's just run with the analysis.' But there was a massive difference in the number of children born to families with a religious affiliation. Many of the Catholic twins we studied had an average family of five children, where other families were having only one or two children.
"We also found that mothers with more education were typically having just one child at an older age. Their reproductive fitness was much lower than their peers who left school as early as possible. Again and again, our analyses for these two factors came back with the same results."
The influence of religion and education in family size may seem an obvious finding -- but what the scientists found really astonishing was that after controlling for these social factors, genetic changes were influencing the three life traits studied.
"Even after we controlled for these social factors, there was still lots of genetically heritable genetic variation in the three life history traits. This is a really unexpected finding."
However, Dr. Owens cautions against linking this work with the possibility of a eugenic program for selective human breeding.
"Looking to the future, I would expect to pick up genetic changes within the ten generations since industrialization. However, what this work doesn't indicate or find is a genetic marker for human reproduction -- so you can't breed for early reproduction from our data. All the traits that we have examined are controlled by interactions between the environment and many genes."
Future work aims to understand more fully the contribution psychological factors make, Dr. Owens says. "We also want to repeat our experiments using twins databases elsewhere, to really put our results into a 'western world' context," he said.
The Australian Twin Registry (ATR) is one of the largest twin databases in the world. It is managed by Professor Nick Martin, another author on the paper, and has taken 30 years to build up. It is normally used to study inherited diseases, but evolutionary biologist Owens was the first to suggest that the data could be used to indicate changes in human evolutionary behavior.
To calculate whether any of these attributes were genetically inherited, the team analyzed lifestyle questionnaires previously gathered by the ATR. The register contains details for 30,000 twins. Of these, 2,710 pairs of identical and non-identical twins satisfied the criteria for inclusion in the team's research.
The data were then assessed using the "classical twin method." This method scores traits on a scale that ranges from wholly genetic through to wholly environmental.
The researchers placed these scores into special "structural models" on a computer, which allowed a number of different influences to be compared within a rigorous statistical framework. The results revealed any correlations between life history traits and reproductive fitness, removing the effect of social and psychological factors.
(Reference: "Natural selection and quantitative genetics of life-history traits in western women: a twin study." Evolution Vol. 55: No. 2, pp 423-435.)
(Editor's Note: The full text of the paper is available online at this URL.)
Imperial College of Science, Technology and Medicine
[Contact: Dr. Ian P.F. Owens, Taslima Khan]