“Speciation, on the other hand, is fairly common.”
It really all depends on the degree of speciation we are talking about. In Darwin’s model, there is no different mechanism to account for a fish becoming a reptile than there is to account for a wolf becoming a dog. It’s the same process, happening over and over for as many billions of years as it would take to explain all the changes in between.
Obviously the second type of “speciation” does seem common as we can see good evidence for common ancestry among a lot of creatures of very similar types, because many can still interbreed to some degree. The other type of speciation, where the process continues until one creature transforms into a radically different type of creature, with what we now know of the probabilities involved, I think we can say only exists in the fertile imagination of Darwinists.
The fossil record shows about 145 million years separating the first fish from the first amphibians plus another 35 million years to the first reptiles.
That's a lot of repetitions of an iterative process, no single repetition of which has to be more than a small baby-step.