Give me an example of these better "genes" that have been accumulated.
Slow down for a minute, Miss, and look at the recent threads on this very forum, and you can find some.
Four examples in the human species, just off the top of my head, are the mutation in Northern Europeans that confers lactose tolerance in adults, the hemoglobin C mutation in the Mossi of West Africa, the Western European mutation from the days of the Black Death that makes carriers largely immune to AIDS, and the mutation in a village in Italy that makes cholesterol a non-issue WRT heart disease.