Is reproductive isolation a tool that helps us distinguish between already different species through the different barriers that prevent organisms of different species from successfully mating and producing fertile offspring, or is it a mechanism of speciation where members of a single species become reproductively isolated from the rest of the population until thousands of generations down the line, their offspring become a species of their own that is unable to breed with members of the original population?
Generally, I’ve seen the first explanation to be the correct definition of reproductive isolation, but in Q29 IMAT2020, it is implied that reproductive isolation was the mechanism through which a new species developed.
Can anybody provide any clarification on this?