I notice In many international cross-country events Is that Ethiopian athletes tend to win and excell quite a bit. Could natural selection be due to this or Is this just coincidence?

I think it is more likely due to the fact that much of Ethiopia is actually at a high altitude. High altitude training is used by serious athletes to improve their performance; this is maximized when the athletes (such as the Ethiopians) are always at a high altitude.

Actually, there may be a legitimate physiological difference:

 

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It's far from resolved, but it's certainly an interesting study system.

I think it is more likely due to the fact that much of Ethiopia is actually at a high altitude. High altitude training is used by serious athletes to improve their performance; this is maximized when the athletes (such as the Ethiopians) are always at a high altitude.

Hence the dominance of Himalayan peoples at marathon running? Coloradans? Andeans?