I’ve been away a bit, thanks allmendream for picking up the ball.
One additional factor I think is assumed by scientists and nt understood by lay people is that when we talk about a new species emerging, it’s not like a car emerging from hte factory. What happens is gradually diminishing mutual fertility with the orginal species.
For an imaginary situation:
At the start: thingy-wampuses 100% fertility among themselves
mutation 1.. taller ones: 90% fertility with the orginal, 100% with each other
mutation 2.. (among the taller ones) big ears: 80% fertility with the original, 90% with taller, normal eared, 100% with each other
mutation 3: short toes among the taller, big eared ones: 70% fertility with the original, 85% with taller, normal eared, normal toed ones, 85% with normal height, large eared ones, 70% with the original.
etc. until fertility with the original becomes zero.
Naturally, in reality the situation is wven more complex, but hopefully this shows the idea.
The closest example I have ever heard of a sort of ‘instant’ speciation was when some insect produced its sex pheromones from a plant chemical precursor. Some insects moved over to a different species of plant with a different chemical precursor leading to the sex pheromones being different between the populations. Instant reproductive isolation, and also differential selective pressure because each little insect will specialize for the plant it preys upon. If you isolated the two different species, no doubt they would still be capable of reproducing together, but it will no longer happen in the wild and the two now different populations will continue to diverge. Eventually transversions in the genomes will make any crosses that can be produced mostly infertile, then totally infertile. Eventually differences can arise making any reproduction between populations impossible.