Yes she could keep breeding her pheasants to create more but that still leaves the problem of introducing new blood. The problem with inbreeding is that it usually leads to infertility, reduced resistance to disease, and genetic and physical issues. Inbreeding can cause certain desirable traits to pop up more frequently in a population, but undesirable traits are also amplified. New blood would widen the gene pool and help avoid these issues. Now your comparison of the differences of pheasant species to chicken breeds is incorrect. With chickens, were talking physical traits the make up a breed, but they are still chickens. We, as humans, select what seperates one breed from another, i.e. the breed standard. The difference between pheasant species is more than skin deep. We are talking about a larger genetic difference that seperates the two species, more than physical appearance. Also, darwins work delt in individuals being bred out of a parent population for long enough that they create a species, or multiple, from that parent population. Now that isnt the whole story as darwin worked on the theory of natural selection and not in the domestication of a species with human influencing the outcomes. Chickens have been bred form jungle fowl long enough the most consider them there own species.
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