A city needs power plants to support just about everything in it, just as a cell needs the mitochondria to support the cell in doing it's job.
Answer: Natural Selection is the (natural) process in nature whereby organisms (plants, animals, etc.) that are better adapted to survive and thrive in their environment, are more likely to survive in order to breed/reproduce and pass on their own genes and characteristics. These characteristics can be physical or behavioral.
Artificial selection is where man, instead of nature, determines which characteristics they would like to hone. In the case of crops, edible, tasty, easy to cultivate crops were chosen above harder to grow crops. Animals kept for meat and other products, were selected based on how much milk, wool, meat, etc. could be produced. Health was also a factor, as sickly animals are troublesome (and potentially unsafe to utilize their products). In the case of domesticated pets, let us consider the dog, originating from domesticated wolves. The more easily trained, protective, better natured wolves were chosen to continue to line of species and the undesirables discarded.
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