What is at issue is the mechanism of evolutionary change. And the Darwinian evolutionary process says something which is also incontrovertible, that like breeds like with variations, that all organisms produce more offspring than can survive into adulthood and reproduce themselves. Those variations which are best able to survive are more likely to survive into adulthood and breed in their turn, so you get evolutionary change like that. That's one of the fundamental mechanisms of evolutionary change. But if you read Darwin himself, he was very clear that there are others as well.
Sexual selection is one, random changes are another.
Darwinian mechanisms are very good for species getting better at what species do, but they're not good at making new species. Darwin himself was very well able to recognize this, which is why the Galapagos became very important. Here were islands very close together populated by species which seemed similar but had particular differences from island to island. Much later when he was looking at the specimens that he'd got from the different islands, particularly at finches of which there are generally agreed to be 13 different species in the different islands, Darwin came to the conclusion that what must have happened is that the original parents of all these finches had come from the mainland, from Ecuador, which is about 400 miles to the east. Once they were on the islands, they bred and they radiated out.