My biggest problem with natural selection as a process to explain speciation is the lack of mutagens. If the genetic dice are being re-rolled every time another individual animal is conceived, thus giving rise to new and potentially useful structures that promote that individual's survival, then...well...where are the mutations, and what's driving the process?
If you roll a six-sided die and get a seven, it's for a reason. I'm open-minded as to what the reason might be, but I am convinced that there must be one. Random mutation alone doesn't seem to me regular enough to result in speciation, particularly when random mutation usually results in fatal defects in individuals.
"explain speciation is the lack of mutagens."
Mutations occur randomly at particular frequencies (typical frequencies are 10^-6 to 10^-7) without mutagens. All mutagens do is increase the frequencies of natural mutations.
IOW you don't need mutagens to get mutations.