It's my understanding that truly catastrophic events have no predictable effect on human evolution because the event is pretty much nonselective (ex: earthquake strikes at night, survivors are determined by which buildings didn't collapse).

But, if the event is such that the local population is severely reduced (and then not immediately replenished by a larger group) , any uncommon traits that randomly survive can rapidly establish themselves in the remaining population if they offer selective reproductive success. Based on human genomic work, it is thought the modern human population passed through such a population bottleneck not all that long ago.

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