A condition of genetic inbreeding. That is, when you take the same genetic template and combine it and combine it with itself over and over and over at a certain point the original integrity of the mathematical template is compromised and breaks down until you start to get random probabilities that were not part of the original designed plan.

That's how the Fallen Angelic races came into being in the first place.

(The Mechanics of Manifestation – CD 1 20:34 min)