Both are demonstrably possible. There is nothing in evolutionary theory to say that the Darwinian process cannot create mechanisms which are no longer Darwinian. A small minority of biologists believe that capacity for directed mutation might have evolved in other organisms, particularly prokaryotes. Pluralism is appropriate in the study of evolution, not of adaptation. In other words, evolutionary processes other than natural selection, such as mutation and neutral drift, do not produce adaptation.
Natural selection can, in principle, explain all known adaptations.
This section is intended to counter a popular creationist argument that certain adaptations are fundamentally impossible to explain by natural selection. This argument is exemplified in a recent book, Darwin's Black Box , by Michael Behe. This argument typically takes one of two forms, both of which are fundamentally related. An organ is too perfect to have evolved by small steps. The vertebrate eye is a favorite example.