That&#x27;s a start, but it&#x27;s not very satisfying.<p>- Ring species. A can mate with B, and B with C, but A cannot mate with C<p>- How do you test it? What about the case where A and B can in molecular terms reproduce (ie the sperm and egg make a viable fetus by IVF) but they can&#x27;t come together naturally because there&#x27;s some behavioural issue (different mating rituals)?<p>- What about things that don&#x27;t have sexual reproduction?