With the availability of better resources like healthcare, food, shelter and hygiene, humans experience fewer selection pressures. Yet, a major analysis involving the genetic blueprints of 150,000 Britons and 60,000 Americans – the first of its kind – has concluded that the variations associated with Alzheimer’s disease, high cholesterol and asthma had appreciably decreased in a mere two generations.

In other words, natural selection could be doing its part in weeding out these conditions.

Seeing Darwinian evolutionary theory at work

The research team at Cambridge and Columbia Universities said that the results of their new study was a demonstration of how the current genomic “revolution” was 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 who was one of the study’s authors.

Using the age of participants’ parents at their death as a proxy to account for the relative lack of older people, researchers found that people with a variation in the ApoE4 gene tended to live shorter lives than those without. That particular gene is linked to Alzheimer’s.

They also discovered that there was a drop in the frequency of a mutation in the CHRNA3 gene in male participants, starting from middle age and above, as compared to those younger. This specific mutation is associated with a tendency for heavy smoking in the men. The implication is that men with that gene tend to die early.

The study, published in the journal PLOS Biology, observed it was “surprising” that just two common gene variation could play such a large role in influencing survival. Moreover, there were fewer variations than expected, meaning they had been naturally “selected out”. Clearly, natural selection is still hard at work here.

“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 research lead Hakhamenesh Mostafavi.

“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.”

In theory, common, but devastating conditions like Alzheimer’s could effectively be “selected out” of the human race in a few thousand years. On the other hand, genetic traits associated with increased chances of survival could be selected for.

Traits considered to be associated with increased chances of survival vary with environment

Interestingly, the study also found that those with a genetic tendency for delayed puberty and child-bearing tended to lived longer – with a one year delay in puberty and child-bearing lowering the death rates by 3 – 4% and 6%, respectively.

However, the researchers cautioned that the kinds of traits considered to be favourable may differ based on environment, which changes with time and location. A trait found to be deleterious in one area may help survival in another – a theory known as balancing selection.

"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," said Mr Mostafavi.

Indeed, an additional phenomenon, “balancing selection”, is at play here. Gene mutations that work against survival may circulate within a population to maintain genetic diversity, while also confering additional benefits. The best example of this is the gene mutation for sickle cell anaemia, which is also associated with additional resistance against the disease Malaria.

Remarkably, the results of another study, posted on BioRxiv, imply that the gene mutations associated with Alzheimer’s disease may also have been the ones which led to humans evolving to have increased neural connectivity and greater intelligence.

Balancing selection applies not only to the gene in question, but to neighbouring genes which are “in genetic linkage”, according to Tobias Lenz, an evolutionary immunogeneticist at the Max-Planck Institute for Evolutionary Biology, in Germany, who investigated this complex phenomenon.

“To some extent, it tells us that we will never be able to get rid of deleterious variation, because of such complex trade-offs,” he says. MIMS

Read more:
More to Alzheimer’s disease than just amyloid plaques
Nursing aide who sexually preyed on frail female Alzheimer’s patients finally convicted
5 unusual genes that create real-life superheroes
5 major Alzheimer’s research findings announced at AAIC 2017

Sources:
http://www.telegraph.co.uk/news/2017/09/05/natural-selection-weeding-outalzheimers-high-cholesterol-asthma/
https://www.yourgenome.org/stories/are-humans-still-evolving
http://nautil.us/blog/why-hasnt-natural-selection-eliminated-heritable-disease
https://www.nature.com/news/alzheimer-s-origins-tied-to-rise-of-human-intelligence-1.17589