Genetic Diversity

There is obvious divergence in body composition between breeds in cattle, pigs, and sheep.[1] Perhaps the most extreme example is the difference in the market pig in the early 1900s compared to the early 2000s. The earlier pig was extremely fat, because fat was a valued product, not only as a source of fat-based by-products, but also because it provided an energy-dense dietary component required by the human consumer who performed very energy-intensive work in the home and in the workplace. Fatty meats provided much of the caloric density required for survival. During the 1900s, with the development of oil-based products and especially as humans automated many of the energy-intensive types of work, demand for fat became limited. Carcass fat is highly heritable, so much progress was made to produce leaner, more muscular animals.

In addition to the genetic selection for muscle mass and against fat mass within breeds, there are breeds of cattle, e.g., Belgian Blue, and sheep, e.g., Callipyge, that are extremely muscular.[2] In pigs, the majority of marketed animals are cross-breeds, and the sires are usually more muscular breeds, e.g., Hampshire or Duroc, or some crossbreed with a considerable contribution from muscular breeds. There have been several experimental selection projects in pigs to study the heritability of fat deposition; these have produced extremely fat pigs (also lightly muscled) in relatively few generations.[3]

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