Basically natural selection weeds out the less suitable organisms for that area. They either got to move out, adapt, or die (thus extinction). With so many organisms on this planet interacting, many species are born and many species change to adapt. An example is the finches Charles Darwin studied. Yes, they are all finches, but their beaks, colors, shapes, and diet are all slightly different from each other. They all have adapted to a suitable life-style for them to survive.
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