What Defines a Specie?

I got to thinking more about "evolution," and it occurs to me that how one defines what constitutes the boundary between species is important. If a puppy is born with a mutation that give it a red nose, and that trait is passed along to the next generation and beyond, does that red nose constitute a new species? I would gather not ... but it got me thinking about what truly does define when one species stops and another begins.

This is important because one of the challenges to the proponents of evolution and natural selection is this: show an example where evidence shows the chain of mutations leading to the creation of a new species. Or show me a laboratory experiment where viruses in a petri dish mutate into a bacteria.

The work to show a common genetic link to some long-past humanoid in Africa does not convince me ... how are we to know that was a different species? It may just have been a very hairy human. But again ... the key to this is the definition of a "species."

Any idea?