Once genes are mixed all over the place as seen in many domestic animals (like cattle) can you (over time) selectively breed out the modern genes to get back to the earlier primitive version of the animal, or is this impossible once genes are mixed?
Not really. You can breed back to something that may look very much like an ancestral form, but you are never going to be to reconstruct it exactly in genetic terms, at least using traditional breeding methods.
If you knew the detailed genetics of the ancestral form, using modern genetic techniques one could in theory perhaps eventually arrive back at an ancestral form, but I think the effort, cost, and time involved would be enormous.
In my Staff Report on evolution of cows, I discuss the only partially successful efforts to restore the extinct ancestor of modern cows, the Aurochs.
With some critters that have been artificially bred to look a certain way (pidgeons, for example), Darwin pointed out that one could find primitive–as opposed to derived–traits appearing with crossbreeding. But as Colibri said, you’ll never get it back to an exact duplicate.
Oh, I should add that that assumes that they were all the same species, originally. In the the case of the pidgeons, the original was the rock pidgeon, I think.
Right now, they are trying to breed back horses to their primitive ancestors.
From wiki:
The Tarpan, Equus caballus gmelini, became extinct in 1880. Its genetic line is lost, but a substitute has been recreated by “breeding back”, crossing living domesticated horses that had features selected as primitive, thanks to the efforts of the brothers Lutz Heck (director of the Berlin zoo) and Heinz Heck (director of Tierpark Munich Hellabrunn). The resulting animal is more properly called the Wild Polish Horse or Konik.
FWIW, in the commentary for Braveheart, Gibson notes that the oxen used to pull the cart in the beginning were rented from a farm trying to breed oxen back to what they would have been like at that time.
If a population of assorted dogs of various breeds are allowed to go feral and interbreed, after a number of generations the population tends to converge to a medium-sized, short-haired, red-brown dog (like a dingo or carolina dog). I’ve heard it’s thought that this is similar to the original ancestral dogs from which modern breeds are descended. The variety we see in dogs today is the result of selectively bred mutations that get masked by dominant genes when intermixed.
I don’t know how close this is genetically, rather than just in appearance, this is to ancestral dogs.
That’s not current work. The Heck brothers “recreated” the Tarpan in 1935. They are the same guys who tried to recreate the Aurochs. The Aurochs-like breed they created is known as Heck cattle, mentioned at the end of my staff report.
I’ve always heard that if you toss goldfish into a pond they’d eventually breed themselves back to their ancestral, Asian carp form. Assuming of course, there aren’t native carp in the pond for them to hybridize with.
Most domestic animals, if allowed to go feral, will eventually “revert” to a form that resembles the original ancestral form, or at least is closer to it. But like I said in my first post, this does not mean that they are genetically the same as the ancestral type.
I think “entropy” must somehow come into play here…
No. Entropy can be thought of as the amount of “order” in a system system. In and of itself, it is rather meaningless. The second law of Thermodynamics gives meaning to entropy in a closed system (no energy added from the outside): The total entropy of an isolated system can never decrease. But a breeding population is not isolated from external energy sources.
You might think of some sort of biological equivalent to the 2nd law of Thermodynamics, but that would not be so much of a “law” as a propensity. A law of nature cannot be violated. A propensity (I made up the use of that as a technical term) is more an observation about what tends to happen, as opposed to what must happen.
I’ve heard a much stronger statement, that in fact all carp in the Americas are decended from domesticated goldfish.