The correct answer is when one population cannot interbreed with the other population as there will be speciation.
Speciation is an evolutionary method via way of means by which a brand new species come into being. A species is a set of organisms that can reproduce with each other to supply fertile offspring and is reproductively remoted from different organisms.
Thus it is clear that when one population cannot interbreed with the other population then one species of organisms becomes two species.
To know more about phylogenetic species refer to the link :
https://brainly.com/question/385240