Selecting for size only...
“Genetic diversity is essential to the health of any species, and variation in characteristics allows the species to adapt to changes in the environment. Human selection has greatly emphasized subspecies diversity, and most domestic species contain a beautiful and useful variety of genetically distinct breeds. There are, however, biological parameters. Animals that are too small, for example, do not reproduce and persist in nature.
Of particular interest in livestock production has been the characteristic of size. The livestock industry has made great progress in increasing the size of production breeds of all species, because bigger is considered better. We know that size limits have been reached, however, when birthing problems, skeletal deformities, and physiological diseases result. Selection for extremely small size can lead to even more pitfalls. This is because many different abnormalities can result in retarded growth. If size is the only selection factor, then undesirable qualities will also be included in future generations.
Twenty years in veterinary practice demonstrated to me an amazing range of anatomical and functional disabilities associated with selection for tiny dogs. Serving as a companion animal is not taxing, but genetic infirmities can still lead to lives of suffering for the animals and heartache for their unsuspecting owners.
Selection of livestock for the single trait of small size makes even less sense, since these animals have real work to do.” ---Excerpt from “Dexters Threatened by Genetic Diversion Gimmick” by Don Bixby, July-August, 1996, ALBC NEWS.