Yes, we have seen it in resistance to diseases. Diseases spread readily in Eurasia, and over time resistance built up to them. When the colonization of the Americas started, these diseases arrived at the same time in a population that hadn't encountered them before and this had no resistance. We don't know how many died, but some estimates go as high as 95% of the indigenous population.

http://en.wikipedia.org/wiki/European_colonization_of_the_Americas#Disease_and_indigenous_population_loss