If you actually go to some tribe out in the jungle you may find they are all either healthy or dead, strong or dead, agile or dead etc etc.
Weak genes die out naturally, mnore so than in a society where the weak are artifically kept alive.
I have another point BUT I'm not 100% on it. Depending on what food is available, the humans in the area will either turn into farmers or stay just gathering bits on the way if there is plenty. This leads on to different skill being developed. That's what I would guess is one of the reasons for the apparent difference in races advances technologically.