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The four foundations theory is one of many theories in the world of horse breeding which is designed to supplement the history of the horse, filling in gaps in knowledge and explaining how horses evolved into their modern, domesticated form. It is important to remember that the four foundations theory is only a theory, and evidence may emerge to contradict it at some point in the future. There are also competing beliefs about the evolution of the modern domestic horse, some of which contradict the four foundations theory.
This theory suggests that all modern domestic horses are descended from one of four foundation stocks, which developed unique physical traits in response to their environments. According to the four foundations theory, modern horses emerged from Europe, Asia, and the Middle East, with the “protohorses” in the four foundations forming the basis of all modern horse breeds, from the Shetland Pony to the Clydesdale.
There is some argument over whether the four foundation breeds were actually separate horse species, or subspecies, meaning that they could have interbred. According to the theory, the four types of protohorses which emerged in Europe were: forest horses, draft horses, oriental horses, and tarpan horses. Each of these horses would have been distinct from the other foundations. Numerous examples which fit into each of these categories can be produced, although people could also argue that the four foundations theory is a back-formation which relies on modern horses to explain ancient horses, rather than the other way around.
Forest horses are also sometimes called warmbloods in discussions of the four foundations theory, with people believing that they adapted to the forested environments of Central Europe. The draft subspecies developed to handle the damp, cold environments of Northern Europe, with a small, stocky body covered in shaggy hair, while the oriental type developed in the dry, arid deserts of the Middle East, becoming tall, slim, and energetic. The tarpan evolved to handle the cold, dry world of Northern Asia, being small, sturdy, and somewhat shaggy.
Proponents of the four foundations theory say that it could explain the incredible diversity of the modern domesticated horse quite neatly, and there is some evidence in remaining wild horse stocks to support it. However, it can be dangerous in biology to make assumptions on the basis of existing modern information alone, as evolution can be extremely complex, and we may be missing a vital key in the puzzle.
@KoiwiGal - It's not that they think horses evolved from four different species. The Four Foundations theory is that horses were domesticated four different times in different places.
Actually the modern theory holds that stallions were only domesticated once, and that wild mares were added to herds multiple times after that, which could account for the wide variety of different body types available in the modern domestic horse.
There were also quite a few sub species of horse that could have interbred with what we consider the horse, like Przewalski's horse which is nearly extinct today, but looks quite a bit like modern domestic horses.
They don't know for sure how it all happened as it is still being studied. But they aren't claiming that the genetics of the horse diverged to the point where they couldn't interbreed and then somehow returned to that state.
I would have thought it was a necessity that the four proto horse types could interbreed, because otherwise how could modern horses interbreed? I think it must be near impossible for an evolutionary path to bring species to the point where they can breed again after going through a period where they can't.
On the other hand, I suppose I'm taking it for granted that all modern domestic horse species can interbreed. I've never been told that they can't although obviously in some instances it wouldn't be a great idea.
Breeding a pony mare with a Shire stallion for example could lead to the mare suffering terribly and probably dying from the birth as the foal will be much too big for her to carry to term and deliver.
Genetically, though I thought they were close enough that it would be possible.
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