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mtsgsd, it is obvious you have no idea how evolution works... It's basically survival of the fittest. Not the strongest, mind you, but of the one best suited to a certain environment.
Suppose we have a rat. This simple rat gets kids (with mrs. rat, of course), and after some breeding and a few generations, we have a few hundred thousand rats, all living in the same environment. Amongst these hundreds of thousands of rats, a few have mutations - small differences in gene structure. This can be as simple as a slightly lighter fur.
Now, if the environment happens to be rather light in color (for example: lots of snow), the rat with lighter fur color will have a slightly higher chance of survival. If the rat then breeds, he will produce kids with a lighter fur too. These will continue to breed, and expand. With their slightly higher chance of survival, the lighter-colored rats will slowly replace the darker-colored rats in the area...
Perhaps the darker ones will be able to find another area where they have a slight advantage: they might "learn" to stay in their hideouts during the day, and only come out at night. Some of these dark-furred rats will develop better night vision, and replace the other rats.
*That* is evolution at work. Small changes leading to a statistically higher chance of survival, eventually replacing the other creatures. And it immediately illustrates why many creatures have similarities: both the light and the darker rats are still basically rats, sharing a lot of similarities. Over the millennia, they'll grow more and more apart, but the basic body functions (and perhaps their overall shape) will remain.
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