The only mechanism in neoDarwinism for introducing novelty of form is genetic mutation. Yet advantageous or beneficial spontaneous genetic mutation remains no more than a hypothetical necessity to the neo-Darwinist theory.
No one has ever observed a spontaneous inheritable genetic mutation that resulted in a changed physical characteristic, aside, that is, from a small group of well-known and usually fatal genetic defects. Because noone has ever observed such an event, noone really knows whether they occur at all and, if so, how often. Because deleterious mutations are known to occur, Darwinists appeal to the statistics of large numbers. If deleterious mutations can occur, then given enough time beneficial mutations must occur.