Natural selection may prefer younger mothers and higher BMI in humen

New study indicates evolution is still acting on contemporary humans, although over many generations and very weakly

As humen continue to evolve, natural selection appears to be favouring higher body mass indicator( BMI) in men and an earlier age for starting a family in girls, research has revealed.

Researchers utilized data from the UK Biobank, a large genetic and health database of half a million British people aged 45 and over, to look at how numerous traits from body mass index to height and birth weight, as well as particular genetic variations associated with such traits, are linked to the number of children individuals had during their lifespan.

But scientists note that the effects are weak and that it will take many generations before significant changes are seen in humen. In addition, they stress it is not clear if natural selection is acting directly or indirectly on the traits.

” We wanted to try to understand what kinds, and[ to] quantify, the evolutionary forces that are affecting contemporary human traits, including height and BMI ,” said Jaleal Sanjak, a co-author of the research from the University of California, Irvine.

” And also characterise whether selection was pushing the population in one direction or another, or favouring intermediate values or extreme values ,” he added.

The results of the study, published in the Proceedings of the National Academy of Science, reveal that for many traits, including height and waist circumference, natural selection appears to be disfavouring extremes.

It also found that while surveys have shown natural selection has disfavoured both very heavy and very light newborns, that selection is now exceedingly weak, and merely seen in females.

” The strength of natural selection is a fraction of what it was, and this birth weight thing is a beautiful instance of that because now neonatal care is so good that you can be very underweight or very overweight and it stimulates no change ,” said Steve Jones, emeritus professor of human genetics at University College London, who was not involved in such studies.

” The interesting thing is not that there are bits of natural selection around- which without question there are. But what is called the opportunity for natural selection has almost disappeared .”

In humen, the team found that natural selection appears to favour a higher body mass index. But with obesity linked to problems with fertility, said Jones,” higher BMI been shown that big muscly hunks are the ones who do better “.

However Sanjak said there might be more to the findings, pointing out that it is difficult to unpick cause and effect.

” The problem here is that the genetic variants that predispose an individual to have a higher BMI also seem to predispose an individual to have more children, that is true. But it could be the case that having more kids has an effect on your BMI .”

He added that it could be that BMI was genetically links between other traits that were under natural selection.

The study, he added, also suggested that natural selection is favouring reduced educational attainment in women.

But, it seems, that is not a straightforward connection, with further analysis disclosing that the result is most likely linked to selection on persons under the age of starting a family, with younger moms less likely to have reached higher levels of education, or less able to reach them once having had children.

The team also showed that starting a family earlier in life appears to be favoured by natural selection in women.

” We understand that having children earlier should mean you have more of them, but the surprising portion was just to find it on the genetic level ,” Sanjak said.

Overall, he said, the results paint an interesting image.” Natural selection is still happening in modern humans- it is observable, we can see it.

” But they are fairly weak impacts and secular tendencies, things due to modern medicine and social change, are likely to be bigger drivers of changes in these traits .”

For example, while natural selection was preferring reduced educational attainment, educational attainment overall was increasing over time.” That is an example of a secular force that is going to swamp the effect of natural selection ,” he said.

Dr Chris Tyler-Smith from the Wellcome Trust Sanger Institute greeted such studies.

” People sometimes ask,’ Are humen still evolving ?’ because advances in society and healthcare now allow people who might in previous centuries have died young to survive and have children ,” he said.

” But, in my view, unless everyone has exactly the same opportunity of having children, we will inevitably continue to evolve .”

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