What happens when a pair of birds that look the same, sound the same, act the same, and live in the same place turn out to be entirely different species? Researchers from the National University of Singapore and the Wildlife Conservation Society recently discovered that what people know to be the Southeast Asian Streak-eared Bulbul is actually two physically identical, but genetically distinct species in disguise. It’s a phenomenon called cryptic speciation, and it’s actually a lot more common in nature than you might think.
To us, cryptic species look and act like doppelgangers; but there has to be a level of recognition that transcends human senses. Otherwise, how would the groups stay distinct? This is where the human definition of a species gets a little fuzzy. In general, animals that breed with each other and produce fertile offspring are considered to be one species—but that’s far from a hard and fast rule. Bison and domestic cattle, for example, will mate successfully, despite the fact that they’re not just different species, but also from different genera. Traits like morphology, habitat, behavior, and bioacoustics can be factored in as well.
In the bulbuls' case, the species in question are the same size, have the same coloration, the same beak, the same vocalizations, and the same tropical terrain. The only visible difference between the two is the color of the eye—a distinction so small, it’s flown under the radar for years. But different individuals of the same species can have discretely-colored eyes. So what makes these birds special? In short: their DNA. By testing two separate wild populations of bulbuls, one in Thailand and one in Myanmar, scientists found that though they share a common ancestor, there's a “deep genome-wide differentiation” that identifies them as unique species. The Thai population will remain the Streak-eared Bulbul; the Myanmar one has been provisionally named the Ayeyawady Bulbul.
While the bulbuls’ level of alikeness is relatively unusual, cryptic speciation is being revealed more and more often in birds thanks to DNA testing. The most notable recent example is the ostrich, which was thought to be a single species for several centuries, but officially became two last year: the Common Ostrich and the Somali Ostrich. Closer to home, it turns out that what we knew as the wren, Troglodytes troglodytes, is now divided into three populations (see here, here and here) that manage to overlap geographically but not interbreed—the textbook definition of speciation, at least, from before we began to sequence genomes.
It can be tough to keep up with these rapidly changing taxonomies, but look on the bright side: Thanks to genetic testing, we have a much better picture of our planet’s amazing biodiversity.