Producing infertile offspring is not necessary for two things to be different species, though. Fertile offspring from two different species is not all that uncommon.<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Hybrid_speciation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Hybrid_speciation</a>