History of the domestic sheep exact line of descent between domestic sheep and their wild ancestors is unclear. The most common hypothesis states that "Ovis aries" is descended from the Asiatic ("O. orientalis") species of mouflon. A few breeds of sheep, such as the Castlemilk Moorit from Scotland, were formed through crossbreeding with wild European mouflon. The urial ("O. vignei") was once thought to have been a forebear of domestic sheep, as they occasionally interbreed with mouflon in the Iranian part of their range. However, the urial, argali ("O. ammon"), and snow sheep ("O. nivicola") have a different number of chromosomes than other Ovis species,
European Endangered Species Programme loss of genetic variability, and accumulation of deleterious mutations. These problems can easily result in loss of original wild traits, and in the expression of heritable abnormalities. If what was once a pure, wild population of animals deteriorates through generations of uncontrolled breeding into inferior or partially domesticated stock, then the animals are no longer suitable for any conservation effort, and the zoos have failed to perform an important educational task. The effects of breeding captive populations of wild animals over periods of many generations have been well studied. Based on these studies and genetic theory, guidelines for breeding such