Talking about dogs and artificial selection, however:
The General writes:
Unfortunately for Darwinists, this analogy (which attempted to show that natural selection and artificial selection were the same) is terribly misleading. It is misleading because in artificial selection the plant and animal breeders enjoy the intelligence and specialized knowledge to select their breeding stock and to protect their changes from natural dangers. Yet, Darwinist natural selection seeks to establish that purposeless natural process can substitute for intelligent design. For this reason, as well as many others, artificial and natural selections are fundamentally different. In addition, artificial selection shows that there are definite limits to the amount of variation that even the most highly skilled breeder can achieve. The breeding of domesticated animals has produced no new species. Interestingly, the fact that breeding can create no new species shows that artificial selection provides powerful testimony against Darwin’s natural selection.
As an example, the reason dogs do not become as large as elephants, much less change into elephants, is not that we have not been breeding long enough. Instead, it is because dogs do not have the genetic capacity for that degree of change, and they stop growing when their genetic limit is reached.
You are correct that artificial and natural selection are fundamentally different. The key difference is that artificial selection starts with a defined goal or purpose, whereas the natural version merely takes what's there and may or may not modify things. As to dogs, you are also probably correct that genomic plasticity of modern breeds is more or less limited, although back-crossing and atavisms are fairly common. However, it isn't for the reason you stated: evolution has limits. The reason dogs at least don't become giants is because 1) once "ideal" breed characteristics were defined, breeders have gone to extraordinary and often draconian lengths to keep their champion bloodlines "pure" - i.e., maintain the desired traits at the desired frequency (which is one of the reasons mutts are often healthier and more stable); and 2) no one ever saw the need or desire to breed elephant-sized dogs.
Interestingly, it would be at least theoretically possible to take a largish natural population of
Canis lupus (assuming you could find a wild-type strain that originated the domestic dog) and set up an artificial breeding program if you wanted elephant-dogs using modern breeding methods. After a large but finite number of generations, always selecting for size, you might be able to generate a humungous breed of giant dogs. Of course, there are likely to be MAJOR structural problems if you're not careful (really really spindly legs that can't support weight, or other bizarreness). And you'd probably also have to have a number of lineages going to cross them down the road to avoid significant in-breeding depression, for instance. After all, given the extant size variation between chihuahua and great dane, for instance, you certainly have one that appears "elephantine" compared to the other. In any event, there's nothing in the
Canis genotype that would preclude it, as far as I know.
In the wild, on the other hand, none of these "sports" would have a chance UNLESS the total selection pressures on the population favored an increase in size. With artificial selection you can obviate or eliminate a lot of the pressures that would otherwise limit something like that (i.e. limited food resources, difficult birth, mate preference, predation (bigger tends to equal slower, in general terms), climatological or other abiotic limiting factors, etc).
Artificial selection, because it shows how vast variation can be produced within a given species, provides a very nice demonstration of the creative power of the "selection" part of the equation. Which is, of course, how Darwin used it.
Oh, by the way, plant breeding can and DOES create new species. All the time.
[This message has been edited by Quetzal, 08-08-2003]
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