Source: Imperial College Of Science, Technology And Medicine
Date: Posted 4/24/2001
New Research Confirms That Natural Selection Is Acting On The Current Human
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. They show that this trend is being passed on from generation to
generation through natural selection - the process where successful genes
pass on to successive generations through evolution.
These findings are different from previous studies of pre-industrialised
populations, where the age at women start their periods is the best
predictor of the number of children that they will eventually produce.
The differences in behaviour are probably due to changes in society that
occurred along side industrialisation, 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
“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
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 behaviour will change
over the next few hundred years.
The influence of social factors on inherited genetic change
Evolutionary biologists have long agreed that the life history of modern
humans has been moulded by evolution due to selection that occurred in the
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
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, he cautions against linking this work with the possibility of a
eugenic programme for selective human breeding.
“Looking to the future, I would expect to pick up genetic changes within the
ten generations since industrialisation. 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
The future work aims to understand more fully, the contribution
psychological factors make, says Dr Owens. “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 Dr Owens was the first to
suggest that the data could be used to indicate changes in human
To calculate whether any of these attributes were genetically inherited, the
team analysed 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 satisfied the criteria for inclusion in the
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
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.
Editor's Note: The original news release can be found at
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