You're a bit muddled in your terms. Darwinian evolution is a collection of ideas which in Darwin's time didn't include mutation. He knew that species change over time, he knew that every group of organisms descended from a common ancestor, he knew that species multiplied by splitting into daughter species, he knew that speciation occurs through gradual processes rather than by saltation (sudden emergence of representatives of a new type), and he gave us one of the mechanisms of evolution: natural selection. Darwin didn't know the origin of new genetic information: mutation. In fact the merging of genetics and natural selection into the neo-Darwinian synthesis didn't happen until around 60 years after Darwin died. You're not getting genetic drift [wikipedia.org] right either.
It is completely accurate to say that the only way species don't evolve is if they go extinct. At the most basic level evolution is simply the change in allele frequency in a population over time. Get a mutation, the allele frequency changes. Natural selection kills something off, ignoring clonal populations that's a change in allele frequency. Whether or not a change is necessary for survival is looking at it wrong. After all, other members of the species are getting along just fine without some specific mutation. A better way of looking at it is to ask if the mutation is compatible with survival. Does the mutation result in a nonviable organism? That's bad. Does it do nothing? Fine. Does it give you an advantage under some or all situations? That's good and as a result you might get more offspring and over time produce a large shift in allele frequency. These changes brought about by mutation and selection (and other evolutionary mechanisms) build up over time and new species inevitably result. There's nothing religious about it.
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