Before Darwin, fossils had already provided good evidence that new species have arisen at many times in the history of life. In 1859, Darwin proposed that new species originate when populations of an existing species diverge over time through the natural selection of small variations. (In the modern version of his theory, new variations are supplied by genetic mutations.) Although Darwin had no evidence for natural selection, it has since been documented in the wild. Nevertheless, biologists have never actually observed the origin of a new species by variation and selection.
In the absence of direct observation, indirect evidence is used to support Darwin’s theory. Ring species are a widely cited example of such indirect evidence. Ring species are chains of geographically adjacent populations, in which each population interbreeds freely within itself but somewhat less freely with those adjacent to it, and the populations at the ends of the chain do not interbreed when they come in contact. By the most common definition of “species,” the populations at the ends of the chain have become separate species.
If a ring species existed it would not be an example of observed speciation, because the chain would already be in place. But a ring species would provide good indirect evidence that speciation can occur as Darwin proposed, with populations diverging gradually in different environments until their descendants can no longer interbreed.
The classic example of a ring species was the herring gull, with populations circling the northern hemisphere. But this example is not what it has been advertised to be. In a 2004 paper titled “The herring gull complex is not a ring species,” German and Dutch biologists concluded:
What earlier authors… regarded as “the herring gull” turned out to be an assemblage of several distinct taxa (argentatus, vegae, smithsonianus), which are not each other’s closest relatives. Our results show that the ring-species model does not adequately describe the evolution of the herring gull group.
Another often cited example was the Ensatina salamander. Populations of this animal circle the mountains around California’s Central Valley. Like the herring gull example, the salamander example is not what it has been advertised to be, but for the opposite reason. Whereas the birds in the first example turned out to be separate species that are not closely related, the salamanders turn out to be members of the same species, Ensatina eschscholtzii. The populations at the ends of the chain are varieties that interbreed to a limited extent. In 1997, Berkeley salamander expert David Wake concluded that “the complex appears to be in a state of incipient species formation.”
Now, “incipient species” is a term that was introduced by Darwin to mean varieties that are predicted to become separate species in the future. And if they do, that would constitute evidence for the origin of new species by a Darwinian mechanism. But what if they don’t go on to form separate species? What if, like the Galápagos finches, they go the other way and start forming hybrids that are hardier than the parents — that is, they merge rather than diverge?
Calling two populations “incipient species” is not empirical evidence, but a theoretical prediction.
None of this has much bearing on intelligent design. The two salamander populations at the ends of the California chain are not distinguished by features that would have to be produced by an intelligent agent. The issue is not ID, but whether the evidence supports Darwin’s theory as much as his followers claim it does.
Photo credit: Herring Gull at Fuglsang Beach, the northern part of Funen, Denmark; Malene Thyssen/Wikicommons.