Ah… you say: “they are simply principles of biology”. Excellent.
You should note in your book that one of the principles of molecular biology is that there is no “line in the sand” - - visible or invisible - - which prevents 2 or more separated populations (from an older common population) eventually becoming distinct and separate populations.
When enough genetic changes have been aggregated that prevents the production of fertile offspring … the game is up. And from that point on, each population can become increasingly different in appearance and behavior … so that a fish starts walking around on the land… so that a hippo starts looking like a whale… and so forth.
So when you say “simple principles of biology” … this would imply that you accept simple principles of biology. But you do not.