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Anonymous
Breeding in small genetic groups can actually increase the strengths of the individuals and improve the genetic stock, if environmental selection is ruthless. Benefits can begin immediately.
An example. A farmer puts two horses in a field. One has weak back legs and the other has poor eyes. Together they have four foals. Of these, one has weak legs, one has weak eyes, one has weak legs & weak eyes. The farmer comes along and _bang_, shoots the weakest horse. Over all the genetic stock in the field has improved.
Sexual reproduction is a way to accumulate harmful mutations in single individuals, and kill them off. Hard on the individual, good for the species. Life is soft, and we are not being selected by a harsh environment, so this doesn't work so well now.
The biggest reason incest is a taboo, is a _social_ reason. Marriage is a way of forming an alliance between two successful family groups. You can't do that if you marry your sister. You lose a priceless opportunity to expand the resources of you genetic stock. In India incest is used to strengthen ties within an extended family and keep small social groups undiluted. Falls on its ass without selection though.
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