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 (see http://www.cdc.gov/malaria/about/biology/sickle_cell.html) as a protective advantage.
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.
There are some other mutation traits which have spread rapidly because they conferred survival advantage. For Europe we have the white gene which enabled people to move north from Africa because fair women could absorb Vitamin D better and produce hips wide enough to yield live young. Now, it is also believed that Neanderthal had fair skin, red hair and blue eyes, so some interbreeding may have helped too.
Another gene is the one that enables adults to drink cows milk without suffering from digestive upset. Getting rid of lactose intolerance was a great boon to farmers in Europe who had cattle. Places like China, America, etc. that lacked large bovines never spread this gene even if it did occasionally occur. Apparently, there is also a genetic basis for survival of the black death in Europe. Perhaps some are aware of other recent genes we could list.