What are these barriers?
Plant and animal breeders have been limited by the barriers for thousands of years. Ernst Mayr call the limit to the amount of genetic variabliity available in a species, "genetic homeostasis".
(This must be my lucky day - thank you for the comment about cordiality. Information involving both a sender and a receiver requirers that I infer one of at least two possible intents of an apparent compliment, so I'll go with the odds and pick your sincerity, which has been observed in abundance in other postings:-)
Cordially