With how pokemon breeding works, whoever they mate with would produce a child that isn't their species. So... How are they common enough to encounter repeatedly in the wild? If it was a case like Nidoran or Volbeat+Illumise where the female can produce either one, it would make more sense, but it's not.
The only way they could reproduce would be with a Ditto. Are ritual ditto hunts a thing for wild pokemon? Also, the same problem applies to Ditto. Shouldn't the Ditto die off from the same problem and they both go extinct? How have any of them survived past their first generation?
https://www.youtube.com/watch?v=NsEIu6-lciA 