Races do evolve. Eurasians evolved immunologic resistances to smallpox, measles and influenza through natural selection as a result of their domestication of useful animals. The reason the Eurasians developed these resistances and other races didn't is because their environment lacked these domesticatable animals since they either hunted them to extinction or they didn't have them in the first place.

These immunologic resistances meant that when Eurasians interacted with other races, they were unknowingly participating in biological warfare. The diseases they sometimes carried that had little effect on them devastated the races that hadn't evolved resistances. This contributed greatly to their colonial efforts.

In summary, the reason Eurasians are boss is because of their geography, not their genome.