The border, obviously, is the point where members of what was formerly one species are no longer able to successfully interbreed due to mutation. Mulism.
This is hardly an invention of creationists, but an internal assumption of evolutionism.
So, as if by magic, in order to prevent evolution, they just stop?
And what has mulism got to do with it.
This is hardly an invention of creationists, but an internal assumption of evolutionism.
And internal assumption? No, it is an easily observable fact.
Google "ring species" and see for yourself:
Ring species provide unusual and valuable situations in which we can observe two species and the intermediate forms connecting them. In a ring species:
- A ring of populations encircles an area of unsuitable habitat.
- At one location in the ring of populations, two distinct forms coexist without interbreeding, and hence are different species.
- Around the rest of the ring, the traits of one of these species change gradually, through intermediate populations, into the traits of the second species.
A ring species, therefore, is a ring of populations in which there is only one place where two distinct species meet. Ernst Mayr called ring species "the perfect demonstration of speciation" because they show a range of intermediate forms between two species. They allow us to use variation in space to infer how changes occurred over time. This approach is especially powerful when we can reconstruct the biogeographical history of a ring species, as has been done in two cases. Source