A new study has highlighted how reproductive biology and human behavior influence the number of children a person might have. This research, which has identified genetic factors that impact human reproduction, has also shown that natural selection has been affecting the human genome for thousands of years and still does today, which has ramifications for human fertility. Many different biological mechanisms and even behavioral characteristics were found to be involved in how fertility varies from one person to another. The findings have been reported in Nature Human Behavior.
"This study is of interest to understanding changes in human reproduction over longer periods of time, reproductive biology and potential links to infertility," said co-corresponding study author Professor Melinda Mills, the Director of Oxford University's Leverhulme Centre for Demographic Science. "It also empirically tests one of the most gripping and fundamental questions asked by scientists across many disciplines and decades: Is there evidence of ongoing natural selection in humans and, if so, what is it and how does it operate?"
This study utilized data biobank data from 785,604 individuals of European ancestry. The researchers revealed 43 regions of the human genome that contain small variations that are linked to reproductive success, which the study authors defined as the number of children ever born (NEB) to a person. These loci impact many things including the timing of puberty, levels of sex hormones such as testosterone, the age that menopause occurs, or whether a person has endometriosis. Behavioral characteristics, like the propensity for taking risks, also seem to have an influence.
There were also some fertility trade-offs. For example, variants in ARHGAP27 that can increase the likelihood of having more kids were also linked to a shorter fertility window during a person's lifetime. So carriers of these variants might be very fertile, but for less time than the average person. Another gene called melanocortin 1 receptor (MC1R), which is linked to red hair color, may also affect reproductive biology. However, the mechanisms that alter hair pigmentation do not seem to be related to fertility.
This research has taught us more about reproductive biology, and could help reveal targets for treating reproductive disorders, noted co-corresponding study author Professor John Perry of the University of Cambridge. "It will also help us better understand the biological mechanisms that link reproductive health to broader health outcomes in men and women."
The researchers also added data from historical genomes to the study, which showed that one region of the genome "has been under selection for thousands of years and remains so today."
There are two genes in this region, called FADS1 and FADS2, which are related to fat synthesis and may have helped Europeans adapt to different diets. This adaptation may still be happening.
"Independent evidence shows the FADS region has been under selection in Europe for thousands of years. It represents the clearest example of a genetic variant with evidence of both historical and ongoing natural selection, though the reason for selection remains unclear," added co-corresponding study author Dr. Iain Mathieson of the University of Pennsylvania.
Sources: University of Oxford, Nature Human Behavior