There are tons of mutations floating around all the time. Some may be slightly harmful. Many are neutral. Some will be helpful. Only the immediately harmful are immediately weeded out. You don't have to be a genius to realize this.
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I'm still waiting for the ' immediately harmful ' tyrants and despots of the world, 'to be weeded out' by evolution.
Accordging to some recent studies of gene distributions in Asians, Ghengis Khan may have set a record for passing on his genes. Somebody back around his time was getting a lot of it all over the place in East and Central Asia. G.K. is the likeliest suspect based upon historical accounts of his appetites, the size of his harem, his travels, etc. What was harmful to practically everyone else's line of descent was great for his own.