Answer:
Because of the natural selection
Step-by-step explanation:
Variation in nature is all we can found out there. Through millions of years nature went with the survival of the fittests, like Darwin said.
When we talk about artificial selection, we talk about humans getting involve on that selection, but we deppend on variation in nature. We can't "create" a new species, but we do can choose the genetic traits we want to prosper and which no.