[Zhang Fei]

I don’t believe anyone raises an argument when it is pointed out that people who have lived at high altitudes (Andes, Himalayas) over generations developed a tolerance for low oxygen levels, or those who have lived in malarial environments developed red blood cells with the sickle cell trait.

Some people think that species grow wings after they're born or something. In reality, strains with useful characteristics for a given environment outbreed those that don't. But there are limits imposed by given form factors. Humans can't survive on a cockroach's calorie intake. Humans can't breathe under water. Tibetans who had no tolerance for high altitudes presumably either left for the lowlands outside of Tibet or died (young) of altitude sickness. In time, the only people left in Tibet were those who had that tolerance, as altitude-tolerant couples begat altitude-tolerant children.

[PapaBear3625]

Tibetans who had no tolerance for high altitudes presumably either left for the lowlands outside of Tibet or died (young) of altitude sickness. In time, the only people left in Tibet were those who had that tolerance, as altitude-tolerant couples begat altitude-tolerant children.

The only people who could be born at high-altitude, where the ones whose mothers could successfully give birth at high altitude.

[BroJoeK]

It's super-important to keep all this talk of DNA diversity in perspective.
Biologically speaking, the “human race” is not just one species, we are a single sub-species, with most of the differences amongst us being no greater than you'd find between typical dog breeds.

Yes, genetically, no two humans are identical and children are born with 60 new mutations, on average, from their parents.
But that's 60 out of THREE BILLION DNA base pairs, and most of those have no effect whatever on us.
So, the total of human DNA diversity amounts to one tenth of one percent of DNA base pairs.
This compares with around two tenths of one percent with Neanderthals and around five percent with Chimpanzees.

Point us that genetic differences amongst humans are relatively small, and even though evolution (or de-evolution if you prefer) continues every day, we are still much more alike than different from each other.