But it doesn't. There are perfect examples of speciation from isolation still living -- with all of the intermediate forms (transitionals, or "missing links") still in place!
Just google "ring species!"
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
Two different colors of the same salamander or warbler do not make for different species any more than two different colors of humans makes for two different species.