There are countless examples in the medical literature on dogs with physical and behavioral problems caused by over selection for a single trait. For example, breathing problems in short-nosed breeds, such as Bulldogs or Pugs, are caused by over selection for a shortened nose. Pointer dogs with extreme nervousness are another example. Pointing ability and nervousness are genetically linked traits. Breeders have known for decades that a fine line exists between a good pointer and a nervous wreck.
Horses also can suffer negative effects with over-selection for both physical and behavioral traits.
White piebald patches in Paint Horses, conformation and jumping ability in Warmbloods, conformation and speed in Thoroughbred race horses, and cow-cutting ability in Quarter Horses are examples of single traits that, when selected to the extreme, can cause problems in other traits. Single-minded selection for rapid weight gain in cattle, pigs, and chickens has already caused serious health and welfare problems. Pigs selected for large amounts of lean meat are so nervous that they often have heart attacks and die during handling or transport. Cattle with double-muscle traits cannot birth normally and must undergo Caesarean sections, and chickens bred for rapid weight gain often develop abnormal behaviors, have weak legs, and peck other chickens. Horse breeders should be careful not to repeat the serious mistakes made with dogs and other animals.