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When a new species takes foot on Earth the previous one doesn't magicaly disappear.

Here is the best example I can think of: Imagine a group of 100 finches flying away from Asia. The finches are coming from a vast environment which allowed them to have varied beaks both sharp and dull. 50 of the finches went landed on an island that contained 90% hard seeds while 50 finches landed on an island 60 miles away that contained 90% soft seeds. You can imagine which finches would dominate each island, in a 100 thousand years the two groups of finches would have evolved in completely different ways and would eventually cease to be able to breed with each other. Therefore, new species comes from same species.

Another, even more simple example is 'Daisy World'. On Daisy World there is always black and white daisies. But, when the sun is brighter the black daisies dominate and when the sun is lessened the white daisies dominate. Quite simple really.