MC Banhammer Posted:

And have we seen that happen? Or are we just looking at two different fossils and saying “it LOOKS like this happened”?

We’ve seen it happen. The cichlid fishes have strong mating preference based on colouration and markings, the ****ers speciate at the drop of a hat every time a mutation comes up that changes that.

There was also a bird species (some kind of warbler, I forget the name) where a mutant had a red breast, whereas the parent population had all black plumage. Red breasts being essentially neither here nor there when it comes to adaptive value the mutation spreads, then they start breeding within themselves. Not yet entirely, but there’s evidence that all-black males see red-breast males as less of a threat to their territory, suggesting reduced breeding between the groups.

Once that starts happening there’s a number of mechanisms that tend to push from reduced mating to complete genetic isolation, and then you have two species going their separate ways. Takes time for any real difference to build up, but it’s a start.