That makes some sense.
OTOH, if you randomly mutate those letters, most of the time you don’t get words. It would seem that random mutations would give you a dictionary of gibberish rather than a dictionary filled with a vocabulary of useful, varied words.
If you take all the combinations possible and compare them with all the combinations of valid words, it would seem there would be far more gibberish cranking out from random mutations. The gibberish would continue to be replicated and eventually, you would not have enough words left to build a viable creature.
Why? If it's harmful gibberish, the organism with that mutation won't survive. If it's neutral gibberish, it might or might not, but who cares? When that organism mates with one of the more numerous of its species that don't have the mutation, the offspring might get the original version rather than the mutated copy.
But then there's the case where the gibberish is actually a useful new word. In last night's Cosmos, they used the example of an arctic bear that had the mutation for white fur. This is obviously not that rare a mutation--white tigers are born on occasion. Where tigers live, white fur would be a disadvantage; but where arctic bears live, it would be very useful. So the white bear is a more successful hunter, lives longer, mates more often. Some of its offspring have the brown fur of its other parent, but some have the white fur because the new word gets copied. The white ones are more successful, mate more often, etc. etc. and presto: polar bears, a new species.