Since species transitions typically take thousands of years minimum, and changes on the order of what creationists would like to see demonstrated may take much much longer, these events don't take place on a time scale short enough for direct observation.
There are examples of populations that are diverging in an incipient speciation event. Two examples I've read about recently are with a species of frog and a species of abalone.
When they do we may find out how it happens.
Until then, no one has seen it ~ they've guessed at it, and speculated about how it can happen, and so forth ~ I think it's going to be something far more complex than so far imagined, and will involve a bit more than the simple transfer of chemical information at the cellular level.