Natives weren't killed off by whites. The flintlocks actually had a lower rate of fire then the spears and arrows that the Natives used!! They were killed off by foreign diseases carried by whites, who occassionly exploited this. Thus, evolution favored whites via better immunity.
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That's an almost complete lie. The better immunity bit is complete bullshit. Like you said, whites EXPLOITED the fact that they had diseases that the Natives never had, and thus could easily die from.