From Wolves to Dogs

Believe it or not, every guard-, lap-, and guide-dog is distantly related to the wild wolf.
Everything from ankle-biting terriers to England Mastiffs are canines, and share a common ancestor. The first step was taken back in the days of natives, when the humans began to toss scraps of food and bones to the less agressive wolves that may have lingered just outside the glow of campfires. These slightly friendly wolves, through the generations, crept closer and closer to mankind. Possibly, when hard winters and game was scarce in the woods, the nicest of the wolves survived on the human's scraps when their more agressive brothers died of starvation.
Humans also adopted pups they might have found on hunting trips, and raised the first tame pet. The canines slowly became part of the native's lives, learning to help man hunt and protect in return for food and care. However, the more independant wolves remained in the wild, fending for themselves.
Everything from ankle-biting terriers to England Mastiffs are canines, and share a common ancestor. The first step was taken back in the days of natives, when the humans began to toss scraps of food and bones to the less agressive wolves that may have lingered just outside the glow of campfires. These slightly friendly wolves, through the generations, crept closer and closer to mankind. Possibly, when hard winters and game was scarce in the woods, the nicest of the wolves survived on the human's scraps when their more agressive brothers died of starvation.
Humans also adopted pups they might have found on hunting trips, and raised the first tame pet. The canines slowly became part of the native's lives, learning to help man hunt and protect in return for food and care. However, the more independant wolves remained in the wild, fending for themselves.

After thousands of years of the wolf living and providing company for humans, small physical changes became more dominant, such as floppy ears, shaggy fur, and shorter snouts. These characteristics may have been chosen by people because it made the animal appear cuter, or the characteristics were random mutations in the wolves that were allowed to continue, whereas it would end up killing the animal in nature.
For example, if a wolf pup was born with shorter legs than usual, it has a higher chance of being killed by another animal. The shorter legs don't allow it to run fast, and it wouldn't be able to escape from danger. This process of survival of the fittest ensured that only healthy animals would live long enough to reproduce, known as natural selection.
If the pup was born as a human pet, however, the humans would protect the wolf from outside danger, possibly because it's stubby legs were simply adorable. The wolf would grow up, and it's offspring may have a similar mutation and shorter legs. Thus, a normally crippling characteristic could be allowed to pass on to future generations, elliminating survival of the fittest
For example, if a wolf pup was born with shorter legs than usual, it has a higher chance of being killed by another animal. The shorter legs don't allow it to run fast, and it wouldn't be able to escape from danger. This process of survival of the fittest ensured that only healthy animals would live long enough to reproduce, known as natural selection.
If the pup was born as a human pet, however, the humans would protect the wolf from outside danger, possibly because it's stubby legs were simply adorable. The wolf would grow up, and it's offspring may have a similar mutation and shorter legs. Thus, a normally crippling characteristic could be allowed to pass on to future generations, elliminating survival of the fittest
The Fox Farm Experiment
In 1959, Dmitry Belyaev founded IC&G. Their goal was to prove that domestication was genetic, and that physical characteristics are somehow linked genetically to behavior changes. Lyudmila Trut, now the leading Reasearcher at IC&G, was working as an intern for Belyaev when the project was first started. She helped choose the 130 foxes from a fur breeding company* in Estonia to be bred and create a lineage of domesticated foxes. She was instructed to search for calm, friendly foxes that responded affectionally (or at least didn't bite or growl) to the humans. These 130 foxes were then taken to Novosibirsk, Siberia, where the main IC&G research facilities were located.
Not all of the foxes were allowed to reproduce, though. Belyaev chose only the tamest foxes to pass along their friendly genes. His foxes were selected for their tameness, not a physical trait or other behavioral characteristic. Natural tamness was the key, so that each generation would become more and more domesticated. This is a process known as selective breeding, as opposed to natural selection.
Not all of the foxes were allowed to reproduce, though. Belyaev chose only the tamest foxes to pass along their friendly genes. His foxes were selected for their tameness, not a physical trait or other behavioral characteristic. Natural tamness was the key, so that each generation would become more and more domesticated. This is a process known as selective breeding, as opposed to natural selection.
*A fur breeding company breeds animals, of a certain desired pelt, together so that they can skin them and sell their fur.