This one is pretty self-explanatory. The most noble bloodlines seem to have bred primarily with their own kin in an attempt to keep their bloodline as pure as possible. We know nowadays that such incestuous behavior can lead to issues with a child's immune system, susceptibility to genetic disorders, and far more horrifying birth defects. Was there ever evidence for this in dynasties anywhere in the world?
As an extension, are there any particularly important histoical figures who may have been affected by this?
Oh absolutely yes there have been. Perhaps the best one I can think of is Charles II of Spain, who reigned at the turn of the 17th-18th centuries. His family tree is a classic example of Habsburg incest - his father was his mother's uncle, for instance, his maternal grandmother being his paternal aunt. And that wasn't the only case of uncle-niece diddling in his family tree, either. Not to mention, literally all of his ancestors on all sides of his family all go back to the offspring of Philip I and Joanna of Castile. That's right, his family tree doesn't continue expanding outward like ours do, it literally went back in on itself. The poor bastard had an extreme example of the infamous Habsburg jaw defect, to the point he could hardly speak, not having learned to do so until he was four years old. He couldn't walk until he was eight, and he was plagued by physical and mental disabilities. He never attended school and his hygiene was poor, even by contemporary standards. Still, though, he was a Habsburg, the last Habsburg ruler of Spain as a matter of fact, and his death at the age of 38 in 1700 triggered a full-blown succession war which lasted over a decade and shaped much of Europe after that point in terms of geopolitics.