which stage is the last stage of speciation?
the populations become adapted to different environments and eventually become so different that they cannot interbreed to produce fertile offspring.
the populations are isolated and thus do not interbreed.
the populations become totally separated from one another.
the populations interbreed successfully.

The populations become adapted to different environments and eventually become so different that they cannot interbreed to produce fertile offspring.

Speciation is the evolution of one species into two different kinds, and when they become isolated, that is the final step, since they cannot interbreed any longer.

it is because according to the process , scientific inquiry is a process with many steps because each step occurs to the scientist and if you just skip a step there would be no steps to scientific inquiry.