One reason that I, and no doubt many others, are unconvinced that simple random mutations and then natural selection, in the Darwinian sense, are enough for an evolution into new species is probably that well established life forms would probably need to pass a missing-link phase that could be likely to be able to survive such natural selection. George Cuvier, who is behind still used reconstruction methods for extinct animals, and also the view that there have been mass extinctions in our evolutionary history denied the possibility even of one species evolving into another! Modern views tell us that it is absolutely impossible that Darwin's theory (of natural selection) is wrong. And I basically agree, but can't believe missing links of evolution would be always able to survive even well enough to be exactly those links towards new species. Why would they be able to leave their ancestors good adaptations, and then just be lucky with survival, not to mention with finding good partners for sexual reproduction. Why would the former species not be to hardy for such a missing link to have any good chance for both survival and reproduction. What can actually be behind their survival?
Could, for example, mass extinctions be a key factor?