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, 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, showing the difference in habitus and ears; not at scale (see the coins). Picture by the American National Science Foundation, available on Flickr.)
All dogs are members of the same species, Canis familiaris (or, if you prefer, C. lupus familiaris):
(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...