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.
- 1500s: Smallpox devastated the Aztec population. It killed most of the Aztec army, the emperor, and 25% of the overall population
- 1500s: Smallpox spread rapidly in the Inca empire and within a few years smallpox claimed between 60% and 90% of the Inca population
- 1633: Plymouth, Massachusetts, the Native Americans were struck by the virus and it wiped out the entire population groups of Native Americans. It reached Lake Ontario in 1636 and the lands of the Iroquois by 1679, killing millions.
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See also the relevant Wikipedia article.