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Readers who were taught about ring species as evidence for evolution in high school are due for a surprise: it now appears that there aren’t any, after all. There were only a few alleged cases to begin with, but now, they’ve all been discredited. The last “good example” of a ring species has just been struck off the list, in a new paper by Miguel Alcaide et al. in Nature
“What’s a ring species?” I hear some of you ask. In a recent post titled, There are no ring species, which is well worth reading, evolutionary biologist Jerry Coyne describes the process whereby ring species supposedly originate:
It works like this: a species expands its range and encounters a roughly round geographic barrier like a valley, the Arctic ice cap, or an uninhabitable plateau. It divides and spreads around the edges of the barrier, so that its range becomes circular as it expands. And as the range begins to form a circle, the populations within it begin to become genetically different as they respond to local selection pressures. But the circle is never interrupted, so while each part of the expanding species becomes genetically different, it still exchanges genes with adjacent populations.
What this causes is a group of populations in which adjacent areas are genetically similar, but become less similar as they become more distant. That’s because the more-distant populations supposedly experience more – different environments, and gene flow between distant populations is attenuated because genes have to flow through all the intervening populations.
At the end, the populations have expanded so far that the ring has “closed”: the species has completely encircled the barrier and the two most genetically diverged populations contact each other. If they are so genetically diverged that they cannot form fertile hybrids, they then appear to be two biological species.
Wikipedia provides a handy definition of a ring species and how it allegedly provides evidence for evolution in its article on ring species:
In biology, a ring species is a connected series of neighbouring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two “end” populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each “linked” population. Such non-breeding, though genetically connected, “end” populations may co-exist in the same region thus closing a “ring”…
Ring species provide important evidence of evolution in that they illustrate what happens over time as populations genetically diverge, and are special because they represent in living populations what normally happens over time between long deceased ancestor populations and living populations, in which the intermediates have become extinct…
Formally, interfertility (ability to interbreed) is not a transitive relation – if A can breed with B, and B can breed with C, it does not follow that A can breed with C…
Ring species also present an interesting case of the species problem, for those who seek to divide the living world into discrete species… The problem… is whether to quantify the whole ring as a single species (despite the fact that not all individuals can interbreed) or to classify each population as a distinct species (despite the fact that it can interbreed with its near neighbours). Ring species illustrate that the species concept is not as clear-cut as it is often thought to be.
There were only a few cases of ring species in Nature to begin with, but as Jerry Coyne acknowledges in his latest post, the last one has now been debunked:
A while back, when I said in the comments of an evolution post that there were no good “ring species,” a few readers asked me what I meant by that. “What about the salamander Ensatina eschscholtzii? Or seagulls in the genus Larus? Aren’t those good ring species?” My answer was that those had been shown not to be ring species in the classic sense, but there was still one species that might be a candidate: the greenish warbler Phylloscopus trochiloides around the Tibetan Plateau.
But now that one, too, has been struck off the list of ring species, leaving no good cases.
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The greenish warbler. Image courtesy of Wikipedia.
The problem with all these alleged instances of “ring species” in Nature is that the formation of these “rings” involved sporadic episodes of geographic isolation between populations, rather than the continuous gene flow involved in making a ring species. That doesn’t matter much for Coyne, because the latest findings still illustrate “how geographic isolation by distance can promote reproductive isolation and speciation.” Coyne is saddened but not dismayed by the discovery that there are no true “ring species” in Nature:
It’s no great loss, though, that we lack good examples, for ring species didn’t really demonstrate any new evolutionary principles. They showed something we already knew — that reproductive isolation is promoted by anything that reduces gene flow between populations. But they showed it in a cool and novel way.
Another textbook icon goes the way of the dodo. How many of my readers remember the herring gull (illustrated at top, courtesy of Wikipedia) from high school? Comments are welcome.