The closest programming analogy for a new gene is probably dropping a new listener/sender on a message bus. It can send messages independently in response to me

The closest programming analogy for a new gene is probably dropping a new listener/sender on a message bus. It can send messages independently in response to messages that were already on the bus before it arrived. If there's a little bit of a shared language (which there is here, since the bus is chemistry itself), that can lead to new behaviors of the system without necessarily breaking anything.