I still do not buy it. I do agree that the environment could play a part in selection. Just the notion that birds in the South are weaker, or from the North stronger is silly. That is carrying the notion too far. It is making more of something where there is nothing there to make anything of. It would make more sense if you shipped birds to Tropical Africa, or to South America. They would not be equipped for the new disease pressures, and it still could be done. South America is full of American and English breeds.
Any isolation in populations is going to make them genetically different. The biggest factor being the difference in the ones doing the selection, and that the population is going in different directions. I can carry two populations in two different directions right here in my own yard. When I crossed them years down the road, I would see some "hybrid" vigor.
I think it was Walt that was telling me once that he had to treat his chicks for coccidia prevention because it's too rampant in the soils where he lives and, that if he didn't, he wouldn't have one bird that would live. I'm wondering if these regional perceptions have something to do with how people handle a proposed threat, such as more of a certain kind of parasite and disease threat due to a hot and humid climate as opposed to someone in the northern climates having to deal more with other types of health issues, but less with those found in a southern or western region.
It's sort of like the cold virus being more prevalent in colder climes because it is encased in a lipid layer that is preserved well in cooler air, thus surviving longer on surfaces and being more easily transmitted in this way. Folks down south just don't have to deal with colds as frequently, so they have not been over medicated for them as people in the north have been....which makes the northern folks even more susceptible to getting the cold viruses. It's a vicious cycle.
If someone treats his flocks as a matter of course in order for them to even survive but someone from a northern climate doesn't do that because the threat is not as real there, then one could perceive that the northern flock has a more hardy constitution because it is able to survive without the aid of medication. That flock may be allowed to form antibodies to various diseases due to low level exposure and antibodies work far better than antibiotics and have a longer lived existence and memory.
In reality, as each generation is medicated for a specific pathogen with the same medication each time, the pathogen is developing a healthier flock of drug resistant offspring, making it even more and more necessary for the flock owner to medicate with stronger and/or different types of drugs to help his flock handle the threat...they just can't form antibodies fast enough to keep up, nor are they even allowed to form antibodies, as medication is the first line of defense. The same goes for parasites, with each generation getting more and more resistant to anthelmintics.
If a large proportion of the flock owners in that region have learned this same behavior to handle the diseases and parasites in their area, then one could say that their flocks/herds have not developed a resistance to disease and parasites, but the diseases and parasites have developed a healthy resistance to the drugs, requiring flocks to be treated more and with stronger drugs. I can't tell you how many times I've been told by folks down south that they HAVE to treat their flocks for coccidia or they all die. I've been told the same thing by southern farmers who keep sheep...that their sheep have to be treated for internal parasites or they will die. It seems to be the general consensus in some areas that their animals will not thrive without chemical assistance.
This seems to be something that has been drilled into them as they got into this or that type of farm animal and are being mentored by more experienced farmers or reading publications addressing these issues in their region. It's very rare nowadays to find farmers breeding and managing flocks and herds with an eye towards disease and parasite resistance in these areas, though it does happen.
Unfortunately, the more a farmer assists his animals instead of breeding and culling for disease/parasite resistant animals, the more they weaken them.
Send that same animal north to a flock or herd that are managed differently because the threat is lower and the first time that animal hits even a low level exposure to disease, they may succumb to it because they have not developed antibodies for it.
I can definitely see where one could start thinking southern flocks may have a weaker immune system, especially if they have experienced this phenomenon first hand.