Memes evolve, and even creationist memes are no exception. It used to be that they denied things like antibiotic resistance in bacteria and pesticide resistance in insects even existed, but now most of the ones that actually end up “debating” sane people seem to agree that “micro-evolution” happens, but “macro-evolution” doesn’t.
The distinction is meaningless, of course, and the argument amounts to nothing more than “I believe in small changes over a small period of time, but not large changes over a large one”. The implication that there is some invisible barrier to speciation, though, still tends to stump many of the people they’re debating, because it’s not necessarily easy to come up with an example that’s simple enough for the wilfully ignorant to understand, and examples of observed speciation in bacteria doesn’t tend to impress people who only barely believe bacteria even exist, much less that it’s meaningful to speak of different species of them.
Ring species, however, are such an example. They demonstrate a gradual process of speciation, and they manage to do it in a way that’s observable in real time.
Put simply, with ring species you have a series of populations along a line or open ring (often a coastline or a river bank, actually). Each of these populations can breed with itself (obviously) and with its slightly different neighboring populations. Perhaps some populations are classified as different subspecies of some overarching species; it’s difficult to point down where one subspecies ends and the next begins, though: the populations basically form a continuum.
The punchline, though: though each population can breed with its neighbors and produce fertile offspring (which, according to a popular definition, makes them the same species), the populations at the ends of the line cannot. They are, in effect, different species.
The canonical example is the herring gull complex around the north pole.1
The ring starts on the shores of the northern North Atlantic with the herring gull Larus argentatus. It can interbreed freely with the American herring gull Larus smithsonianus,2 which occurs, surprisingly, across North America.
The North American herring gull can interbreed with the Vega gull Larus vegae, which is a subspecies of the East Siberian gull (the only other subspecies is the Larus vegae mongolicus, which isn’t relevant to our story). That in turn can interbreed with Heuglin’s gull, Larus heuglini, which can interbreed with the Siberian lesser black-backed gull Larus fuscus heuglini, concluding our trip across northern Siberia.
The Siberian lesser black-backed gull can interbreed with the lesser black-backed gull Larus fuscus, which bring us back to Europe, mostly around the Baltic Sea, though they share some territory with the herring gull.3
These gulls form a species continuum, as you can see, but the herring gull cannot interbreed with the lesser black-backed gull.
It’s hard to tell where one species ends and the next begins, but it’s undeniable that the herring gull and the lesser black-backed gull are different species.
This is just one mechanism of speciation (allopatric speciation; these gulls inhabit much the same niches, so this is nearly entirely down to genetic drift, too), but it’s a plainly obvious one.4
Of course, most creationists would just reply “but they’re all still birds”, which is a level of ignorance against which little can be done.
Some of the more literate ones might point out that not only are they all birds, they’re all still in the genus Larus, moving their non-existent barrier to speciation to a barrier to “genusiation”, but that’s no more to the point than to point out that all new growth on an old tree seems to happen at the bud level, with only new twigs being born, and that no new massive boughs have been added to it in decades.
(Incidentally, there’s an interesting parallel in linguistics, with the dialect or language continuum. Flanders is a good example of it, with most of its many, many dialects being intelligible to its immediate geographic neighbors, but people on one end of the region not being able to understand a damn thing people on the other are saying. It can probably be argued that the Limburgs dialects make Dutch form a language continuum with German.)
1 I’m aware of the Liebers/de Knijff/Helbig paper, and I slightly disagree with its conclusion that the Larus species complex does not represent a ring species. Regardless of how it was formed, it’s a ring now. There may be contexts in which it’s not useful to speak of it as a ring species, but that isn’t relevant here.
If you want to argue about this, by all means do.
2 And, indeed, is almost indistinguishable from it to the untrained eye. The American herring gull is a bit bigger, though.
3 The taxonomic story is actually a bit more complex than that, obviously. For instance, the gulls in the eastern part of the range of the Heuglin’s gull are often considered a separate subspecies (Larus heuglini taimyrensis, or the Taimyr Gull), but some people believe these to just be the result of interbreeding between Heuglin’s gulls and Vega gulls; and of course most species names are in dispute: the Vega gull is sometimes classified as Larus argentatus vegae, and people seem to be confused over what the Siberian lesser black-backed gull actually is.
This is something for taxonomists to squabble over, though, and I’m not convinced it’s a productive use of anyone’s time.
4 If the Liebers paper makes you nervous, there are other, less controversial examples of ring species, including Ensatina salamanders around Central Valley in California, and greenish warblers around the Himalayas.