I was watching a Nova eppisode about Darwin, and as you may or may not know, he married, and farthered 4-5 children with his first cousin… And his oldest daughter died in her teen’s.
Also if you know about the hapsburgs . In order to keep the family as the royal overheads of switzerland, they married and fostered children, cousins, brothers and siters… But in the end, everybody turned sterie, and reproduction without new “blood”, was not possible.
As you see on pictures of the hapsburg’s, they have strong characteristics; Large noses, ears and chin bones.
All this is enherited by their mothers and farthers, and so the circle continues, emphasising the characteristics, and not evolving anywhere… Without turning sterile, and letting the inbreeding continue, the result would be a weak offspring, with no new characteristics, that is not well adapted, or is imbalanced in the environment. This is very unhealthy, and brings the ecosystem out of balance.
That is why i think that it is evolution itself that incorporates a protection against this, with sterilizaion of individuals, when breeding inside a “to close” branch of the family tree….