Inbreeding doesn't generally cause any issues. It can bring issues to light, IF there are issues in the genes of the family line. When the family has no issues, like most reptiles, then it's not going to spontaneously cause an issue to erupt.
Many labs that breed rodents in-house have had the same colony without any introduced blood for years and years. The rats are all fine. Those are even mammals. They simply had all the issues eliminated early on in the colony's life.
The frog pond is a great example also.
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