You must be tired of reading my cribs so I thought that I will perk you up a bit by telling you about...frogs. I know what you are thinking - 'How does this guy get such splendid ideas? What next? Millipedes?' Let's just say it is god's gift and leave it at that.
About those frogs. In The Ancestor's Tale, Richard Dawkins writes:
Microhyla (sometimes confused with Gastrophryne ) is a genus of small frogs, the narrowmouthed frogs. There are several species, including two in North America: the eastern narrowmouth Microhyla carolinensis, and the Great Plains narrowmouth Microhyla olivacea. These two are so closely related that they occasionally hybridise in nature. The eastern narrowmouth’s range extends down the east coast from the Carolinas to Florida, and west until half way across Texas and Oklahoma. The Great Plains narrowmouth extends from Baja California in the west , as far north as northern Missouri. Its range is therefore a western mirror of the eastern narrowmouth’s and it might as well be called the western narrowmouth. The important point is that their ranges meet in the middle: there is an overlap zone running up the eastern half of Texas and into Oklahoma. As I said, hybrids are occasionally found in this overlap zone, but mostly the frogs distinguish just as well as herpetologists do. This is what justifies our calling them two different species.
The most conspicuous difference between the two species lies in their mating calls. Both are squeaky buzzes but they differ in duration and predominant pitch. This difference is clearest in the zone of overlap where they meet. In areas where the two species never meet, the calls are more similar to each other. In areas close to the zone of overlap but not quite in it, the calls are more different. But the maximum difference is in the zone of overlap itself. Something is pushing the two species apart in the overlap zone. One reason for this could be due to what ecologists call competitive exclusion. Dawkins writes:
This phenomenon, where two species differ from each other more when they overlap than when they don’t , is called ‘character displacement’ or ‘reverse cline’. It is easy to generalise from biological species to cases where any class of entities differ more when they encounter one another than when they are alone. The human parallels are tempting, but I shall resist. As authors used to say, this is left as an exercise for the reader.
PS: On a related note, know how social media magnify hierarchies and collapse social distances?