"If evolution is real, why aren't we still evolving?" That's one of the main arguments against evolution that you might hear people sling around. Evolution is an extremely gradual process that's all but invisible to the casual observer, but if you know where to look, the evidence shows that it is still happening. We've seen it in the rise of antibiotic-resistant bacteria and in how generations of fish and lizards respond to changing environments. And now a genetic study has found evidence of natural selection at work in the genomes of over 200,000 people.

Evolution works through the basic principle of natural selection. Random mutations often emerge in an animal's genes, and if they turn out to help that individual survive longer, it'll be more likely to procreate and pass on its genes. Eventually, the most beneficial genes will rise to the top and spread throughout a population, while individuals carrying "bad" genes will slowly be weeded out.

Conducted by researchers at Columbia University, a new study analyzed the genomes of 60,000 US citizens of European ancestry, and 150,000 people in Britain. With a sample size of 210,000 people in total, the researchers were able to track how specific mutations rose and fell across generations, which genes impacted survival rates, and roughly which traits were becoming more or less common in the population. Since the UK database didn't have as many genomes from older people, the researchers approximated the results by taking into account the age of death of the participants' parents.

Out of all of the genetic mutations the researchers studied, two in particular stood out as linked to survival rates. Women carrying one or two copies of a gene called ApoE4, which has been associated with Alzheimer's, appear on average to die much younger than those without it, causing a drastic drop-off of the frequency of ApoE4 in women over the age of 70. Meanwhile, the frequency of a mutation in a gene called CHRNA3 (which is linked to heavy smoking) drops off around middle age in men.

The researchers were surprised to find only two mutations to be so directly linked to survival rates, leading them to conclude that perhaps natural selection had already taken care of similar variations.

"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," says Molly Przeworski, co-author of the study.

Measuring the evolution of specific physical traits is a tougher process, since dozens or hundreds of individual mutations can play a part in determining the likelihood of each trait. So the team collated sets of mutations that are linked to 42 chosen traits, such as height and body mass index (BMI), and then worked backwards to estimate a person's traits according to their genetics.

Using this method, the team found sets of traits that could be linked to longer or shorter lifespans. People predisposed to high cholesterol, high BMI, heart disease and, to a lesser extent, asthma, were more likely to die younger, while delayed onset of puberty seems to correlate with a longer life. The death rate was three to four percent lower for men and women who started puberty a year later than average, while for women, a one-year childbearing delay lowered that rate by six percent.

It may be fairly subtle, but the researchers point to the study as potential evidence of evolution hard at work in humans today. But environmental factors play a part too, and progress isn't always a straight line forward.

"The environment is constantly changing," says Hakhamenesh Mostafavi, lead author of the study. "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."

The research was published in the journal PLOS Biology.