In a fascinating discovery that counters a common theory that human evolution has slowed to a crawl or even stopped in modern humans, a study examining data from an international genomics project describes the past 40,000 years as a time of supercharged evolutionary change, driven by exponential population growth and cultural shifts.
Thanks to stunning advances in sequencing and deciphering DNA in recent years, scientists had begun uncovering, one by one, genes that boost evolutionary fitness. These variants, which emerged after the Stone Age, seemed to help populations better combat infectious organisms, survive frigid temperatures, or otherwise adapt to local conditions.
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Also, I would think that in a larger community it would take longer for a given genetic change to propagate from one person to the rest of the community, unless it is classically dominant. So there would be little visible effect for many generations. But then it might show up among a larger fraction of the population rapidly once the probabilities of two parents both having the gene achieve a sufficient level. If it's a 'good' change, then all is well. But if it's a 'bad' change, then it could cause a large scale problem.