Some have suggested modern medicine has frozen human evolution. With advanced medicine and reproductive technologies, the vast majority of humans enjoy lifespans long enough to pass on their genes to the next generation.

But a new study suggests natural selection is still altering the human genome in subtle ways. When researchers surveyed the genomes of 210,000 people from the United States and Britain, they found genetic anomalies linked to Alzheimer's disease and heavy smoking were less common among those with longer lifespans. Scientists also found those who lived longer were less likely to carry genetic mutations linked with heart disease, high cholesterol, obesity, and asthma.

The findings -- detailed this week in the journal PLoS Biology -- suggest humans are still influenced by natural sections, as those with longer lifespans are more likely to pass along their mutation-free genes.

"It's a subtle signal, but we find genetic evidence that natural selection is happening in modern human populations," Joseph Pickrel, an evolutionary geneticist at Columbia University and the New York Genome Center, said in a news release.

Advances in genomic analysis techniques have allowed scientists to track the rise and fall of genetic signatures across large groups of people.

In the latest survey, researchers found a sharp drop in the prevalence of the ApoE4 gene, linked with Alzheimer's, among women over the age of 70. They also found a significant decrease in the frequency of the CHRNA3 gene, linked with smoking among men.

That only two genetic mutations were strongly correlated with length of lifespan suggests other variations have been purged from the population through natural selection.

"It may be that men who don't carry these harmful mutations can have more children, or that men and women who live longer can help with their grandchildren, improving their chance of survival," said Molly Przeworski, an evolutionary biologist at Columbia.

Researchers also found changes in genetic variants linked with fertility, but said environmental factors can also influence a population's shifting genome. Such genomic shifts may not be beneficial in the future.

"The environment is constantly changing," said Hakhamenesh Mostafavi, a grad student at Columbia. "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."

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