Explain why reproductive isolation usually must occur before a population splits into tow distinct species?

The zoological definition of a species is a population that can form a fertile offspring.





As long as two populations are similar enough genetically, there will be gene flow between them, and they will not become separate species; they will share a common gene pool. If they are separated long enough, mutations and selective pressures will change their genetics enough to prevent their producing fertile (if any) offspring.Explain why reproductive isolation usually must occur before a population splits into tow distinct species?
Once reproductive isolation occurs, the two distinct reproducing populations begin evolving away from each other due to random chance. Eventually, sufficient change will have occurred that the two populations cannot reproduce together. In a single population with random reproduction amongst the members, no suset diverges enough to become disyinct.