n and allows for more positive selection than a small one.
"The second is environmental change - our diets have changed, we are in radically new environments," he added. "With a large population size comes lots of new diseases."
However, geneticist Professor Steve Jones of University College London said suggesting a large population size could increase the speed of evolution was "a contentious issue".
"Once a population gets above a very small size it is not very clear if its ability to respond to natural selection depends on size," he told BBC News.
"The general picture that evolution has speeded up in the last 10,000 years as we change from, to put it bluntly, being animals to being humans is clearly true," he explained. "To suggest it is happening at this instant, I would suggest, is probably wrong."
He said natural selection needed difference - either in the ability to stay alive or in the number of offspring born.
"The fundamental observation is that this difference has gone," said Professor Jones.
"At the moment we are in an evolutionary interval. We are in between two storms. One storm has more or less blown itself out, the storm of farming.
"The question is whether we are going to stay in the calms or whether another great storm will start. And if there is one, I would say it is most certainly to do with epidemic disease."
How they did it
The study looked specifically at genetic variations called "single nucleotide polymorphisms," or SNPs. These are single-point mutations, or changes, in the genetic sequence of DNA on chromosomes.
If the mutation is advantageous then it will spread rapidly in the population, along with DNA on either side of the mutation.
The authors argued that if the same chromosome from numerous people had a segment with an identical pattern of SNPs this would indicate that the segment of the chromosome had not been broken up (recombined) recently.
Therefore, a gene on that segment of chromosome must have evolved recently and fast, they believe. If it had evolved long ago, the chromosome would have broken up and recombined.
Article from: http://news.bbc.co.uk/2/hi/science/nature/7132794.stm