from Darwin, Origin of Species, page 284:
...the difficulty lies in understanding how such correlated modifications of structure could have been slowly accumulated by natural selection.
This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. Thus, breeders of cattle wish the flesh and fat to be well marbled together; the animal has been slaughtered, but the breeder has gone with confidence to the same stock and has succeeded. ..
In the chart about incidence of tt bulls in various breeds, the highest percentage was in the Hereford breed.
http://homepage.usask.ca/~schmutz/leptin.html
Couldn't some grad student look at this across other breeds, especially the British breeds and see how the expression relates to management?
These questions and observations are not new, and I refer to Dr. Clarke from his book Cattle Problems Explained, from 1880.
http://books.google.com/books?id=kDFFAAAAYAAJ&dq=cattle%20problems%20explained&pg=PA164#v=onepage&q=cattle%20problems%20explained&f=false
CHAPTER XVIII.
The Saving Of Muscle In Fattening Cattle.
Exercise and Juicy Meat versus Waste and Degeneration of the Muscles.
The practice of Mr. John D. Gillette, of Logan Co., 111., and others, in exercising their cattle, including the famous prize steers of 1877, led to much more discussion than would have taken place if he had failed to carry off the prizes at Chicago. But the superior quality of the beef produced by his out-door feeding, under the exercise incident to it, suggests a little further inquiry as to the causes of such a juicy and superior quality in his grade Short-horns, or any other cattle, there having been many other successes in feeding under such or similar management.
In many instances,
prize cattle in the London market and elsewhere, have been affected with '' fatty degeneration," or degeneration of the muscles; and, as this condition is met with in stall-fed cattle, while cattle having moderate exercise are not so affected, the origin of fatty degeneration requires some thought. It appears most conspicuously in a degenerated condition of the muscles that are necessarily used during exercise, suggesting the idea that inactivity gives rise to it, as this disorder is known to result from disuse, or inaction, in other parts of the system. It is necessary, first, to see how the size of the muscles is maintained, so that causes and consequences may be compared in ascertaining why the size and substance are reduced, or changed, under certain artificial conditions. Motion in the muscles causes wearing change; wear gives occasion for renewal, causing a demand for nutritive blood supply, in proportion to motion and waste.
Many fat cattle have sufficient vigor of muscle and circulation to escape this disorganizing process—the fatty degeneration of the muscles. But others are seriously affected by it. The disorganization of the fibres and fascicles of the muscles results from disuse and inaction in the legs or other affected parts of the animal; this inaction itself resulting from reduced or prohibited exercise.
In alluding to the practice of Mr. John D. Gillette, and others of Central Illinois, in fattening cattle in their groves and pasture lots, thus allowing them to indulge in considerable voluntary exercise, it was stated that other successes from similar management had been observed. For instance,
Hereford steers have been, and probably still are, frequently fatted and finished off in England on good grass, in open pastures, and without night penning; and the juicy quality of their beef is abundantly well known in the London and other markets of that country. On a more extended scale, Short-horn grades and fine common steers and heifers, in hundreds of herds, and many thousands of instances, fatten rapidly and well on the ample, open Western ranges of rich grass, fully maintaining their muscular proportion by adequate daily exercise. And many steers are fatted during winter weather in ample yards, provided with simple sheds, these cattle voluntarily seeking shelter or indulging in gentle activity, as they require.