Sunday Times (Sri Lanka)

Human natural selection is adding to ‘an epidemic of nearsighte­dness’

Half the world will suffer from the condition by 2050

- By Ian Randall © Daily Mail, London

Natural selection among humans is adding to the ' epidemic' of nearsighte­dness, with each successive generation in the UK gaining more than 100,000 extra cases.

It is estimated that around half of the world's population — some 4.9 billion people — will suffer from the distant visual impairment by the middle of the century. Much of the problem is env i ronmental — with increased screen time and not enough spent outdoors using our long-distance vision often blamed.

However, US experts have found that many of the genetic variants that increase the risk of nearsighte­dness, or myopia, are also associated with reproducti­ve benefits. Thus, those with these genes are likely to have more children, and from a younger age, increasing the relative prevalence of myopia-causing genes in the population.

' We found that the rapid increase in the prevalence of myopia is in part due to natural selection for myopia-associated mutations,' paper author and evolutiona­ry biologist Jianzhi Zhang of the University of Michigan said. This, he added, is 'not because myopia is beneficial

— but because these mutations are also associated with reproducti­ve advantages that are unrelated to myopia.'

In their study, Professor Zhang and colleague Erping Long examined the genetic and medical data of 63,185 individual­s aged between 40–69 who volunteere­d to participat­e in the UK Biobank study. This long-term research project — which includes the accumulati­on of biological samples into a large repository — seeks to identify the genetic and environmen­tal factors that contribute to the developmen­t of disease.

Dividing the subjects into six groups based on age, the team found that the prevalence of myopia increased from 24.4 to 41.0 per cent between those born between 1940– 44 and 1965– 69. When corrected for the effects of age on eyesight, the researcher­s found that this is the equivalent of the likelihood of myopia at age 40 increasing from 30.3 to 43.5 per cent over the 25 year time span. Examining genetic variations known to be associated with an increased risk of nearsighte­dness in the DNA of the subjects, Professor Zhang and Dr Long found 25 variants that appear to have become more common in younger generation­s.

Nearsighte­d individual­s are more likely to have worse reproducti­ve success — defined by having less children and later in life — which suggests that natural selection acts against having the condition of myopia, the duo reported. However, they also found that nearly half of the genes that are linked to an increased risk of myopia and are becoming more common among the population were also associated with better reproducti­ve success.

Specifical­ly, eight genes were found to be linked to people giving birth to their first child at a lower age, and five variants whose presence appears to be associated with having a larger number of children overall. It is not clear how exactly these genes are related to an increase in reproducti­ve success, Professor Zhang said.

'One possibilit­y is that [these genetic variants] affect both myopia and reproducti­on,' he explained. 'Another possibilit­y is that the myopia- associated mutation happens to be located next to a reproducti­on-associated mutation in the human genome such that [the former] hitchhikes with the reproducti­on-associated mutation when the latter is selected by natural selection.' The genetic factor is significan­t, but not the primary cause of the growing incidence of nearsighte­dness, the researcher­s explained.

The full findings of the study were published in the journal National Science Review.

 ??  ?? The prevalence of myopia has increased by 13.2 per cent in 25 years. Much of this increase is environmen­tal — with screen time often blamed
The prevalence of myopia has increased by 13.2 per cent in 25 years. Much of this increase is environmen­tal — with screen time often blamed

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