Why is reproductive isolation required for speciation to occur ?

When a single species changed into two or more separate species, is called speciation. Speciation can take place due to reproductive isolation.  In reproductive isolation, the separated species no longer can interbreed among themselves. This is happened due to mating differences, sterility or environmental barriers which finally results into the adaptive splitting into two species.

if populations can be interbred, they are considered one species is the right answer!

good luck and have a wonderful day!

not too sure but maybe in the hot springs.