Though the probability of a billion or so mutations simultaneously turning out gorilla-like in a bear zygote is virtually impossible, it wouldn't change the physical laws that determine the science of genetics. Evolution isn't even involved because it is the study of populations. If you flipped a billion heads in a row, you might want to re-evaluate your religious stance, or buy a lotto ticket, but it wouldn't change the mathematics of probability.
If you change the "genetic language" as Lam mentions, though, we are no longer talking about mutation probabilities. You would need to come up with entirely new chemical and genetic mechanisms to explain the transistion from old complex entity to new complex entity. Or, give up and go with the God theory, like we did before modern science.
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