A cline describes a smooth gradient of adaptive characteristics across a line of organisms. Some of the most dramatic examples of clines in the animal kingdom are the so-called "ring species." These species, such as the Ensatina salamanders of California and the Larus gulls of the arctic, form a cline that reaches around a natural barrier and meets itself on the opposite end.
What makes ring species such dramatic examples of clines is that while breeding is continuous along the cline, the individuals at either end of the cline cannot or will not breed with each other. This technically makes them different species, though genes flow freely through the entire population by way of the intermediaries.
Larus gulls ring the North Pole in a cline that stretches from Iceland, across northern Europe, along the north coasts of Russia and Canada to Greenland. The Icelandic and Scottish variety of gull is known as the British lesser black-backed gull, and the Greenland variety is called the herring gull. Where these two meet, around the North Sea, they behave as if they were different species and do not interbreed, though breeding is possible between each intermediate species.
Another example of a biological cline is in the Ensatina variety of salamanders. These salamanders live in the mountains of California and are separated by the Central Valley. Together, the subspecies form a ring around the valley and meet in the south. There, while breeding between variants is possible, it is rare.