ONE of the first insights into the origin of species was the case of Darwin’s finches. Actually, they are not finches, but members of another avian group, the tanagers. But the pertinent point is that these birds, which live on the Galapagos Islands and were first made known to science in the 1830s, by Charles Darwin, have different-shaped beaks adapted to different food stuffs even though they all seem to be descended from the same South American ancestor. Some spear insects with thin, sharp beaks. Some have heavy, blunt beaks for cracking seeds open. Some, of intermediate beak geometry, peck at cactuses. One has even worked out how to use cactus spines to extract insect grubs from rotten wood.

Darwin’s finches are an example of adaptive radiation, in which a single species splits into several that are able to occupy different ecological niches. Biologists have, however, long debated whether niche-specialisation is by itself enough to promote such radiation. Many think the incipient species need to be cut off from one another geographically as well as ecologically, in order to stop the changes brought about by local natural selection being swamped by genes flowing in from elsewhere. In the case of an archipelago, such as the Galapagos, that would be easy. On a single island, it might be harder.

A study just published in Evolution, by Kathryn Langin of Colorado State University, sheds light on the matter. Dr Langin looked at such an island, Santa Cruz, off the coast of California. It has an area of 250km2. Much of this is covered by oak woodland, but there are also three large stands of pine. Both habitats are home to an indigenous species of bird, the island scrub jay. Using techniques unavailable to Darwin, Dr Langin has sought to establish whether the jays in the two types of habitat, though members of the same species at the moment, keep themselves to themselves in ways that might cause them to become separate in the future.

Over the course of a three year study she and her colleagues captured 565 of the island’s scrub jays – about a quarter of the population. They measured each bird, paying particular attention to its beak. They took a blood sample, ringed the creature and then released it. They also observed where each bird lived, which birds mated with which, and who the offspring of these unions were. That, plus genetic data harvested from the blood samples, enabled them to construct family trees.

Birds that lived in the pine forests, they found, had different shaped bills from those that inhabited oak woods. Pine-dwellers had long, shallow beaks. Oak-dwellers, short, deep ones. This makes sense. Extracting seeds and hidden creepy-crawlies from pine cones requires a beak that can reach between the cone’s scales. Acorns, by contrast, need to be hammered open. Nor was it a matter of birds being born with a beak shape at random, and then seeking out a suitable habitat. The maximum distance a fledged bird lived, as an adult, from its natal nest, was a kilometre for males, and four for females. That rarely resulted in a change of ecosystem.

Moreover, beak shape did not come about at random. It was actually highly heritable – an obvious necessity if it is to be the basis of speciation. And males and females with similar beak shapes tended to mate with each other, rather than with those of different beak morphology. Gene flow between the two types of jay is, in other words, limited.

Whether it is so limited that the groups will eventually go their separate ways remains to be seen – though probably by generations of zoologists to come rather than those of the present day, for the process is likely to be long-winded. But regardless of what the future holds, Dr Langin’s study is an important contribution in the here and now to the debate about how speciation happens.