If no species ever gives birth to another species then all life on Earth must be the same species. Saying that it happens very slowly doesn't change the fact that speciation has occurred. You describe changes within members of species and say that explains speciation. It doesn't. Human beings have been breeding dogs for thousands of years and introduced great variation in the species, yet they are all dogs. All of Darwin's famous finches are the same species of finch. The gulls in your excerpt from Dawkins are all gulls-- the reason that the ones on the far ends don't breed is that they live too far apart.

Following Dawkins' logic, all creatures on Earth can interbreed, since they are all related. Such turns out not to be the case.

To give one example, let us consider the bat. Evolution would say that the dactyls gradually grew longer until they became wings. So for millions of generations some species of mammal must have existed that had limbs that were too elongated for walking and too short for flight. How, precisely, did this long vanished creature survive?