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A fishy game of tag
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DESPITE its name, Darwin’s masterful explanation of the mechanism of evolution did not truly tackle the question of how species originate. It showed how they adapt to their circumstances, which may lead to populations of the same species diverging in their characteristics. But how a species actually divides into two, non-interbreeding daughter species is glossed over.
One way such speciation might happen is when two geographically separate populations that are derived from a single species meet. If hybrids resulting from the meeting are less fit than the purebred offspring of either parent population, that would encourage those parent populations to evolve barriers, such as distinctive markings, which stopped further interbreeding. Once this has happened the two populations would definitely be regarded as separate species. Beforehand, though, specieshood is rather in the eye of the beholder. This is the case, for example, for bream and roach—two fish that text books generally describe as species, but which are well ...
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