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Alzheimer’s disease, high cholesterol and asthma are steadily being “weeded out” of the human gene pool by natural selection, the first major study of its kind has revealed.

Analysis of the genetic blueprints of 150,000 Britons and 60,000 Americans found the variations associated with the illnesses had noticeably declined in the space of just two generations.

Researchers believe men with Alzheimer’s tend to have fewer children, and that both men and women with the condition are less able to look after their grandchildren.

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This affects survival and limits the proliferation of those faulty genes.

At the same time, genetic traits associated with good health and increased chances of survival are more likely to be passed on, becoming increasingly common in the human gene pool.

It means that, in theory, common devastating illnesses such as Alzheimer’s could be effectively bred out of the human species within a few thousand years.

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The research team at Cambridge and Columbia Universities said their new study showed how the current genomic “revolution” was now enabling them to see Darwinian evolutionary theory play out in practice.

"It's a subtle signal, but we find genetic evidence that natural selection is happening in modern human populations," said Joseph Pickrell, an evolutionary geneticist at Columbia University and one of the study’s authors.

The researchers analysed the complete genetic make-up of 210,000 people from the UK Biobank and the Kaiser Permanente in California, using the age of participants’ parents at their death as a proxy to account for a relative lack of older people in the study.

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They noticed that people with a variation in the ApoE4 gene, linked to Alzheimer’s, tended to die well before those without it.

Another significant trend captured by the study was a drop, starting in middle age, in the frequency of a mutation in the CHRNA3 gene associated with heavy smoking in men, indicating a higher death rate in that group.

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The study described as “surprising” that just two common gene mutations could play such a large role in influencing survival, and that the fact there were not more mutations indicated they had been naturally “selected out”.

“Men can father children even at old age, and even if a tiny fraction of them do so, over time this may be enough of an effect for selection to “see” and act on,” said Hakhamenesh Mostafavi, who led the research.

“For example, if men with ApoE4 have 0.1% fewer children on average than men without them, this would be enough for these variants to be removed quickly by natural selection.”

Published in the journal PLOS Biology, the study also found that people genetically predisposed to delayed puberty and child-bearing lived longer.

A one-year puberty delay lowered the death rate by three to four percent in both men and women, while a one-year childbearing delay lowered the death rate by six percent in women.

The research team said the results are evidence that genetic variants that influence fertility are evolving in some U.S. and Britain populations.

But they cautioned that environment also plays a role, meaning that traits that are desirable now may not be in other populations or in the future.

"The environment is constantly changing," said the study's lead author, said Mr Mostafavi.

"A trait associated with a longer lifespan in one population today may no longer be helpful several generations from now or even in other modern day populations."