Types of breeds...

Landrace breeds - "Landraces....are local populations of animals that are consistent enough to be considered breeds, but are more variable in appearance than are standardized breeds. They also lack the formal definition and organization that is typical of standardized breeds.....A combination of natural and human 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 of 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 geographical 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 careful cultivation of the diversity within breed parameters, the genetic integrity of the landrace is protected even as it becomes a standardized breed.

Standardized breeds - Historically, most livestock breeds began as landraces and then became standardized breeds. As breeders organized, they agreed upon a description (or "standard") of the breed and then began to select their animals towards this ideal. In practice, this means greater uniformity in behavior and performance. Genetic diversity may have been reduced, but predictability was gained....

Standardized breeds are what most people think of when they consider purebred livestock. Human selection has played the primary role in the development of standardized breeds, though natural selection has sometimes played a part as well. The breed standard defines the breed, and it is this criterion by which individual animals are evaluated. Breeding to a standard emphasizes a relatively narrow range of variation, usually less than the variation found in a landrace. Genetic isolation of the standardized breed is thus established by breeding practices rather than by the geographic or cultural isolation typical of landraces. For example, most standardized breeds limit inclusion to those animals with two registered parents.....Conservation of standardized breeds (as well as landraces) requires that the purity of the breed be protected from crossbreeding. The diversity within the breed’s genetic parameters must also be conserved. The two forces of predictability and variability tug against one another, and striking a balance between them is the goal of breed conservation.....A group of animals must breed true to a distinguishing type to be a true breed."

From A Conservation Breeding Handbook, by D. Phillip Sponenberg and Carolyn J. Christman, (pub. by ALBC, 1995), pp.6-9: