“When an isolated sub population can no longer breed with the larger population, you have speciation by definition. “- js
Not exactly - Different species have actual differences in genetics and behavior.
If I seperate out a colony of white rats, and prevent them from interbreeding with any other rats, have I created a new species? Simply no.
-And without further mutation, they will *always* be the same species of rat as the original, and be capable of interbreeding.
The ability to breed successfully, producing fertile offspring is an essential definition of species. A finch will always produce a finch but over generations, one population of finch may no longer successfully breed with another. Horses and burros are close relatives but different species with the mule offspring only very rarely being fertile.
BTW there are several separate “rat” species.