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DNA is like code that holds the instructions for your body. It determines all of your traits. Some scientists even argue that your personality is genetic. If a mutation in DNA occurs, it can affect how your body works. Some mutations are harmful, and can lead to things like cancer, while others (more rarely) can actually be helpful.
Mutations are any change in DNA. This can be an insertion (DNA is added), a deletion (DNA is removed), or a substitution (DNA strands swap places). Things that cause mutations are called mutagens, including UV light, X-rays, and some specific chemicals.
Mutations are usually thought of as harmful, but this is not necessarily the case. Although rarer, mutations can be helpful. These helpful mutations increase the organism's likelihood of survival or reproducing, and fix to the population via natural selection over a period of time. An example of a good mutation would be the one that made wolves less cautious, allowing them to get food from humans. Harmful ones (and good ones, occasionally) are usually eliminated.
Mutations can affect whole entire sections of chromosomes or just a few base pairs. Somatic mutations are those that affect body cells, these cannot be passed off to offspring. Germline mutations affect sex cells (egg or sperm) and will affect nearly every cell in the child's body. This is the basis of evolution. A canis lupus with a mutation that makes it produce less stress hormones can pass the mutated stress gene to it's children. This could fix to the population, and make a subspecies of calm wolves.
Mutations help living things evolve. Migrations, natural selection, and heredity help mutations spread across the population. About 50 million years ago (mya), the ancestor to many mammals, the miacis, roamed the Earth. About 15mya the miacis evolved into the tomarctus, and many other things as well. From there, the tomarctus became the canis lupus, which then became subspecies like the canis lupus rufus and the canis lupus domesticus (domesticated dog).
Co-evolution is a special type of evolution in which two species affect each other's evolution. (humans and dogs!!!) We helped dogs evolve by selectively breeding them to our needs, and caring for them as pups when they were abandoned by their wolf families.
When food was scarce, the bravest wolves wouldn't be afraid to approach the human camps to scavenge for food. These wolves wouldn't starve in the cold of the ice age, unlike their less courageous cousins. This is the start of domestication. Eventually, these friendly wolves would begin to settle in with the humans for a steady food supply, and the humans would put the wolves to work for them. In order for maximum efficiency, we would've selectively bred them to our needs, thus furthering their separation from wolves. (For example, only the kindest, or smartest, or X-est of the litter could contribute to the gene pool.)
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<Picture of dog and human friends drinking lattes at Starbucks reminiscing on their relationship. Dog says, "Yeah, we go *way* back">
Those dogs with mutations that made them "friendly", producing less stress hormones, were able to survive better than other wolves because they were not afraid to approach humans, who provided them with food in times of difficulty. Those "friendly" mutations, along with human's selective breeding, have caused the wolf to morph into the modern-day domesticated dog.