There is a pair of European grasshopper species, Chorthippus brunneus and C. biguttulus, which are so similar that even expert entomologists can’t tell them apart, yet they never cross-breed in the wild although they sometimes meet. This defines them to be ‘good species’. But experiments have shown that you need only allow a female to hear the mating call of a male of her own species caged nearby and she will happily mate with a male of the wrong species, ‘thinking’, one is tempted to say, that he is the singer. When this happens, healthy and fertile hybrids are produced.
