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
Genetic homeostasis is not a barrier to speciation. (Lerner coined the term, btw.) It means that populations have a tendency toward a mean fitness. That mean can still change via genetic drift, for example.
Artificial selection by plant and animal breeders for specific traits within species shows a continuous change over time. A recent post Best in Breed may interest you. Supposed limits in breeding of yesteryear are easily surpassed today.