Along rivers in Tennessee and Georgia, scientists have been studying brownish-orange spiders, called Anelosimus studiosusm (image), that make cobwebby nests “anywhere from the size of a golf ball to the size of a Volkswagen Beetle,” researcher Dr. Jonathan Pruitt says. The individual spiders are only the size of a pencil eraser, but they form organized groups that can catch prey ranging from fruit flies to small vertebrates. “We have found carcasses of rats and birds inside their colonies,” Dr. Pruitt says. Unlike most spiders, which are solitary, these social spiders work together in groups. Now new research shows that they evolve together in groups, too. Mention the term “group selection” among some groups of evolutionary biologists and you won’t be invited back to the party. But Dr. Pruitt, at the University of Pittsburgh, and Dr. Charles Goodnight, at the University of Vermont, have been studying generations of these Anelosimus spiders — and have gathered the first-ever experimental evidence that group selection can fundamentally shape collective traits in wild populations. Their results were published online on October 1, 2014 in Nature. “Biologists have never shown an adaptation in nature which is clearly attributable to group selection,” Dr. Goodnight said. “Our paper is that demonstration.” In his 1859 masterpiece, “On the Origin of Species,” Charles Darwin puzzled over how ants could — generation after generation — produce workers that would serve the colony — but were sterile. Evolution by natural selection has often been understood to work at the level of the organism: the traits of an individual determine whether it will survive and reproduce. How could these sterile ants persist in nature, he wondered, if they didn’t reproduce?
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