Four types of livestock and poultry breeds occur in North America: landraces, standardized breeds, industrial stocks, and feral breeds. These differ according to their history and selection, the relative level of uniformity versus variability, and the presence or absence of a formal structure to organize and promote them. While the majority of North American breeds (and endangered breeds) are standardized, a discussion of the different types is useful in explaining how the breeds were developed and how each should be conserved.
Landraces, (not to be confused with the Swedish Landrace swine breed) are local populations of animals that are consistent enough to be considered breeds, but are more variable in appearance than are standardized breeds.
Individual landrace breeds are unique due to founder effect, isolation, and environmental adaptation. Founder effect refers to the accidents of history that led to introduction of certain types of animals to new areas. These “founders” are the sole genetic base of descendant populations…
A combination of human and natural selection has shaped the evolution of landrace breeds. Natural selection and geographical isolation have created genetic consistency and adaptation to the local environment. Traits such as parasite and disease resistance, reproductive efficiency, and longevity have also resulted. Human selection is of somewhat less importance. In fact, human selection in one part of a landrace population may be counteracted by different human selection in another part. Color is one example- the Holt line of Piney Woods cattle is usually white park or colorsided roan with black ears, while the Conway line is red and white in various combinations. Both individual herds have lost some color variants (gaining uniformity in color in the process), but the landrace breed has not.
Landrace breeds generally lack the formal support of a breed association, and they survive as distinct populations due to geographic and cultural isolation. If communication among breeders increases, and a network of breeders is organized, the landrace may benefit by greater geographical distribution and more secure numerical status. This process can, however, result in selection for greater uniformity across the population and diminish the presence of some of the original variants. If, instead, there is a careful cultivation of the diversity within breed parameters, the genetic integrity of the landrace can be protected even as it becomes a standardized breed.
Thanks again to the American Livestock Breeds Conservancy for permission to use this passage from this wonderful book.