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Given there is no other species that can do the job better.

How much can you change a species to fulfill a specific task. The evolution of horses was pretty extreme from what I remember. Are those drastic changes a thing that can happen with just selective breeding or are there more conditions required ? I am asking the rule and not the example but real life examples of domesticated species would illustrate well.

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  • $\begingroup$ corrected, true but part of their traits were selected to make better mounts no ? $\endgroup$ – Armind Sep 11 '17 at 17:45
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    $\begingroup$ you could just wiki "selective breeding" ...... if all you wanted was the rule $\endgroup$ – anon Sep 11 '17 at 17:57
  • $\begingroup$ You may be interested in the question How fast could a directed breeding program turn another Earth species intelligent? $\endgroup$ – a CVn Sep 12 '17 at 11:22
  • $\begingroup$ The Russian silver fox domestication program had significant success in just 20 generations. It really depends on what your goal is. No (realistic)amount of selective breeding will make a pig fly, for example. $\endgroup$ – Jason K Sep 12 '17 at 14:11
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Charles Darwin wrote a thousand-page book titled The Variation of Animals and Plants under Domestication (1868), as a sort of supporting material for his thesis of natural evolution by common descent with modifications under natural selection. It is available online courtesy of the Darwin Online project.

Good examples of spectacular results are, among plants, maize, and, among animals, dogs.

Maize

Maize, Zea mays, commonly called corn in America, "is a large grain plant first domesticated by indigenous peoples in southern Mexico about 10,000 years ago" (Wikipedia). Its wild ancestor was not known until the 1930s, when two botanists, the Russian Nikolai Ivanovich Vavilov and the American George Beadle identified the teosinte, Z. mays parviglumis, which is a small bushy plant. It looks nothing like maize:

Wild teosinte and domestic maize

(Wild teosinte and domestic maize, showing the difference in habitus and ears; not at scale (see the coins). Picture by the American National Science Foundation, available on Flickr.)

Dogs

All dogs are members of the same species, Canis familiaris (or, if you prefer, C. lupus familiaris):

A Newfoundland and a Chihuahua
(source: wikimedia.org)

(Two dogs, members of the same species: a Newfoundland (left) and a Chihuahua (right). Photo by The Cat, available on Wikimedia under the CC-BY-SA-3.0 license.)

The extraordinary radiation of dogs happened within no more than 15000 years; the vast majority of breeds are much more recent.

The general rule

It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows.

There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

(Charles Darwin, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, 1861)

To realize the power of selection, consider that all four-footed animals are descended from one common ancestor species; all those diverse forms, from the tiny Paedophryne amauensis frog (7.7 millimeters long, the smallest known vertebrate) to the enormous blue whale (29900 millimeters long), including humans, are descended from one ancestral species under natural selection in about 380 million years.

Artificial selection works much faster than natural selection because it is goal-oriented and less forgiving. The limits of artificial selection are given only by the (in)ability of humans to continue a project over long periods of time. With stubborn determination we could duplicate the results of natural selection in not more than one hundredth of the time...

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    $\begingroup$ Yeah, all you have to do is look at dog and cat shows to see how far you can take selective breeding. Also, look at the produce industry. We breed for specific fungus resistances and other traits in our crops. $\endgroup$ – ShadoCat Sep 11 '17 at 18:32
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    $\begingroup$ Another spectacular example is Brassica oleracea "a plant species that includes many common foods as cultivars, including cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, savoy, kohlrabi, and gai lan." en.wikipedia.org/wiki/Brassica_oleracea $\endgroup$ – ohwilleke Sep 11 '17 at 19:24
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Perhaps one way of answering this rather broad question is to consider what can’t be achieved by selective breeding.

A pathway of incremental improvements must exist between the original state of the life form and the required state, each of which must be viable. If this is not possible then there is no way to achieve that change by evolution.

An example the Laryngeal nerve. It would be more efficient if the laryngeal nerve did not pass under the aortic arch. But there is no way for evolution to disconnect and reconnect the vagus nerve in this way because there is no pathway of incremental improvements to achieve this. The issue is most starkly shown in the case of the giraffe which has exactly the same arrangement.

Laryngeal and vagus nerves

Natural selection (and artificial selection by breeding) are also restricted by basic biochemistry and physics. This makes it difficult or impossible to implement some things such as discontinuous structures where an organism is composed of physically distinct parts such as piston rings on a piston in a petrol engine. Chemical constraints also make it difficult or impossible to make a wide range on materials such as steel plate armour or titanium bones.

Hence we do not see any large animals with wheels evolving by natural or artificial selection.

Also, what constitutes a “species” is not always clearly distinct. There are so called ring species of gulls that inhabit a linked series of territories around the arctic from northern Europe, Siberia, northern Canada and Greenland. Each species can interbreed with those from an adjacent territory but cannot breed with those from opposite sides of the ring.

By common convention dogs are considered a “species”, but if we take the definition of a species as a group of organisms that interbred naturally there are issues. Great Danes and Chihuahuas do not interbred naturally so might, by this measure, be considered separate species. It could be argued that they could interbreed by artificial means, but if this used as a measure of what a species is then lions and tigers should also be considered the same species as they can be interbred by artificial means.

Not trying to be difficult here just trying to illustrate how common concepts and conventions can sometime oversimplify or distort a more complex reality.

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Consider how evolution works:

  • You have a species which fits into an ecological niche. Those individuals who fit better are more likely to procreate, so their genes will be more prominent in the next generation. This affects both individuals with an advantageous combination of existing genes, and those with the rare beneficial mutation.
  • The niche changes over time. A predator gets better, an ice age comes, etc. And again the individuals who fit better procreate.

Try it for enough generations and the Indohyus becomes a whale ...

Your selective breeding works on the same principle. Human intervention and human priorities replace the more generic "ability to survive and procreate" as the condition which governs the next generation. Consider some dogs with severe health problems, they prosper because of human preferences.

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Oh boy,

If given unlimited time, there is no extent to selective breeding. The trick is, the amount of time it takes to generate desirable mutations that can be passed on and further refined.

You could eventually selectively breed cows with lizard skin given enough time.

So the rule is: mutation -> selection = evolution

The dairy cow is one such freak: over a millenia of selective breeding has created a creature that would die in the wild. They are breed to be docile (non -aggressive) and over produce milk. They litterally feel pain if not milked frequently enough.

We accelerate mutation by inserting other genes into an animal through various methods.

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  • $\begingroup$ All mammals feel pain if not "milked" frequently enough. Ask any woman who has recently given birth and voluntarily or involuntarily is dealing with sudden weaning. $\endgroup$ – Cyn Feb 28 at 21:12
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It would depend on the variety of genes and how they are expressed within that species to begin with as well as the frequency of mutation among that population.

Say there is a species of animal that is excellent at digging due to broad, tough appendages that function similarly to a shovel.

If there is variance among the traits of the population, for example you find that some of these animals have narrower shovel appendages that are better for digging in small crevices while others have broader concave shovel appendages better for moving large quantities of earth (I am absolutely butchering my technical terms from ecology 101 but hey, I'm not an ecologist.)

You could then selectively breed the narrow shovel animals with other narrow shovel animals to bring out more of that trait and create a breed of the species more specified towards excavating small crevices(and vice-versa with the broad shovel animals).

For good examples of the full power of selective breeding, look at dogs. The bloodhound is a result of selective breeding and is highly specialized at tracking through scent.

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