QUANTA MAGAZINE: Elusive Form of Evolution Seen in Spiders A study of spider colonies supports a controversial idea in evolution — that natural selection can act on communities as well as on individuals.

As a rule, spiders are antisocial. They hunt alone, zealously defend their webs from other spiders, and sometimes even eat their mates. “Cannibalism and territoriality comes naturally to Arachnida, even during sex,” said Jonathan Pruitt, a behavioral ecologist at the University of Pittsburgh. But a handful of the more than 40,000 known arachnid species on the planet have learned to rein in that aggression. Like ants or bees, they cooperate for the good of the group.

For example, so-called tangle-web spiders form bands of 1,000 or more to spin webs that stretch for hundreds of yards, entrapping flies, small birds and “virtually any invertebrate imaginable,” Pruitt said. Smaller groups of a few dozen work together “like a pride of lions,” he said; some of the spiders hunt for prey, while others rear the colony’s young.

The spiders present a puzzle to evolutionary biologists. According to ordinary Darwinian natural selection, only the fittest individuals will pass on their genes. But if that’s the case, why do tangle-web spiders act in ways that might conflict with an individual’s drive to outcompete its neighbors? A spider that defends the nest might put itself at personal risk, jeopardizing its chances of producing offspring. And a spider that rears the young might have to wait to eat until the hunters are sated, so it might go hungry. These are not behaviors that would be expected to enhance an individual’s fitness.

Biologists have long argued over the question of how natural selection can promote the evolution of traits that are good for the group, but not necessarily for the individual. Scientists have developed a number of mathematical models to attempt to explain the phenomenon. According to one model, known as kin selection, highly related organisms such as bees and ants can develop altruistic behavior — for example, many females forgo reproduction in order to raise the queen’s brood — because they will still pass down their genes indirectly, through the queen.

But despite its altruistic appearance, kin selection is selfish — it helps an individual’s genes to survive. Can natural selection promote truly unselfish traits, behaviors that are good for the group, but not necessarily to the benefit of individuals (or their immediate kin)? Some evolutionary models predict that it can, but while these models have been successfully tested in the lab, they have been studied only indirectly in nature.

Now, however, a new study of Anelosimus studiosus, a species of tangle-web spiders,published this week in Nature, suggests that evolution does indeed work at the level of the group. If certain groups of animals are more productive than others — that is, if they produce more progeny — then evolution will tend to favor the traits that make such fecundity possible. According to Pruitt, the findings are the first to provide direct evidence that natural selection can drive the evolution of a group trait in the wild.

полный текст статьи, опубликованной в  QUANTA MAGAZINE: Elusive Form of Evolution Seen in Spiders A study of spider colonies supports a controversial idea in evolution — that natural selection can act on communities as well as on individuals.