Most fatal mutations are fatal before conception or before birth.
Branching to a new species is unlikely or impossible in a large population that is continuously interbreeding. Speciation requires an isolating mechanism.
A mutation may not be fatal until the system that is regulated by the code is highly stressed. Auto-Immune genetic defects work that way. There is not enough time, even if the mutating edits happen to let the creature function. It has been demonstrated by scientific research that Darwin's Finches were not mutations outside of their original code, which means that environmental pressures do not create new code, rather they reach to existing code to see if the population can survive. If there is no code to help the population to adapt enough, they go extinct.
We see this every year as more and more creatures devolve to the point where they are not as versatile as they once were, and go extinct.
Breeding has shown us that reproduction does not create new code. It isolates certain expressions. The isolation always does the same thing until the "blue blooded" little critters grow huge noses and ears, then die from weak cell structure (defective DNA). Unless they breed back into the less mutated population in time.
Bottom line, science show us devolution on a massive scale, within mankind's genetic code, as well as the animal kingdom. If bacteria could evolve via Darwinian Evolution, there would be no more life on earth. The limits were coded in intentionally by a sovereign Creator.