How does a ring species illustrate speciation in action?

Speciation occurs at terminal population groups

Explanation:

A ring species represents a group of organism that has moved beyond a geographic barrier to produce a ring shaped distribution. This population is contiguous and there exists no barrier to gene flow anywhere in the ring except at the terminal populations.  

Speciation is demonstrated in the terminal population has become sympatric and it does not interbreed even if they are connected by chain of intergrading or interbreeding populations

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands