A species is a group of individuals that can mate to produce fertile offspring – but there are some exceptions to this rule. In a ring species, for example, some populations within the species cannot successfully interbreed with others.
- Can humans and chimpanzees interbreed?
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Ring species are those that are distributed in a ring. For instance, ensatinas (Ensatina eschscholtzii) are salamanders that live along North America’s Pacific Coast, from Canada to Mexico – but they form a ring around California’s Central Valley. To the north of the valley, the population splits in two, with one chain of populations moving down the eastern side of Central Valley, and the other chain moving down the western side. These separate chains of populations meet again in southern California, closing the ring.
While the populations to the east and west of the valley could interbreed to produce fertile offspring, they cannot interbreed where they meet in the south.
This is why ensatinas are called a ring species: all populations are technically the same species, as most populations within the species can reproduce successfully, but two populations (the ones that close the ring) cannot.
On the other side of the Pacific, the greenish warbler (Phylloscopus trochiloides) is another example of a ring species. This little bird lives in forests across northern and central Asia, and its distribution forms a ring around the high-altitude Tibetan Plateau. Where the populations meet in Siberia, to the north of the Plateau, they cannot interbreed – but other populations in the ring can.
Both greenish warblers and ensatinas show gradual changes in their DNA, morphology and behaviour across the populations that form their rings. This means that by the time the populations that close their respective rings meet, they have measurably different DNA, morphology and behaviour, and this is why they cannot successfully interbreed.
When these genetic, morphological and behavioural differences are caused by geographical barriers (such as California’s Central Valley or the Tibetan Plateau) it has a technical name: allopatric speciation.
This is the most common way that one species splits into two. And this means that ring species are rather special: they show us evolution in action. Ring species are also a great example of how our definition of a species can sometimes be too clean-cut for what really happens in nature.
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