In a brutal way
Eating and being eaten is as much a part of nature as camouflage and deception. All of this is interdependent and allows both predators and prey to refine their arsenals. But does the varying success of the hunters also lead to the specific division of their potential victims?
Given the abundance of species of beetles that inhabit the earth, an American biologist once suggested that God must have had a special soft spot for this group of insects when he created the world. And indeed, the legions of insects make up a very large proportion of today's animal biodiversity. They also owe their evolutionary success to a large extent to their flexibility and their rapid reproductive rate, with which they can react quickly and easily to environmental changes.
If they conquer new territory - an island that has just emerged or just a plant that has just migrated into their habitat - they can often adapt well to these unknown conditions. And with a bit of luck, a new species will develop over time, whose representatives are specially adapted to these ecological resources and make better use of them than their former conspecifics. Arguably the most famous result of this process, known as adaptive radiation, is the Galapagos Darwin's finches.
But what drives adaptive radiation? One of the main factors is the competition for food and habitat, because if there are too many competing for a specific fruit or for specific brood cavities, a few individuals begin to set themselves apart: they look for alternatives and, if successful, they are often killed with numerous offspring and am Possibly rewarded with its own species status at the end.
But that is obviously not the only driving force behind evolution, as Patrik Nosil and Bernard Crespi of Simon Fraser University in Burnaby, Canada, have now discovered. They looked at specimens of the North American stick insect Timema cristinae, which may be at the beginning of its adaptive radiation and thus on the verge of splitting into two species. They could all still mate fertilely with each other, so that a biologically flawless dividing line cannot yet be drawn - their genetic differences are within the intraspecific framework.
But apparently, these kerfe are already clearly developing, because they appear in different forms depending on the preferred leafy green: those species members who prefer to feast on the needle-shaped leaves of the rose plant Adenostoma fasciculatum have a clear white stripe pattern her otherwise dark green body. The consorts that feed on the broad leaves of Ceanothus spinosus have only faintly indicated stripes or no stripes at all - their green appears brighter and they grow larger.
Depending on the location in the bushes visited, this coloring naturally camouflages the insects with varying degrees of effectiveness, as they can sometimes be spotted more or less easily by predators. Couldn't this feeding pressure also contribute its mite to the divergence of the stick insects? The researchers tested this in the field by releasing the same number of scares from both variants onto several of the two preferred plants. But while some of the mixed populations were protected from hungry birds by nets, their fellow populations were left to the rigors of nature.
Protected Timema cristinae naturally had above-average survival rates - regardless of whether an adenostoma type was moving on a Ceanothus bush or, correspondingly, a Ceanothus mantis was moving on an adenostoma growth. At least predatory forces are not driving evolutionary selection here.
The picture looked completely different on the freely accessible plants: Here, the predators could pluck their food from the branches unhindered. Of course, the birds first ate those stick insects whose appearance did not at all match the forced host plant. Ceanothus insects on Adenostoma fasciculatum, for example, were discovered and eaten much earlier than their Adenostoma conspecifics, which were also crawling around there, which would have been more likely to reproduce as a result.
So far, so simple, but nature does not drive the divergence of the two stocks through this selection strategy alone. Because the Ceanothus spinosus survived the longest on Ceanothus spinosus, whose green was the brightest and whose stripes were completely absent. Conversely, the same was true for the Adenostoma representatives: on their host plant, the darkest coloration and clearest pattern offered the greatest protection.
In both cases, the extremists had the best chance. And since the different populations prefer to mate with each other anyway and avoid the different types of relatives, it should only be a matter of time before they finally part ways. God and the world would have one more insect.