Spiders Know the Meaning of Survival of the Fittest

It seems that a certain species of spider knows the meaning of survival of the fittest, according to a new study.

The controversial idea of "group selection," is that evolution will favor traits that increase the success of a group of plants or animals, rather than just focusing on the survival of an individual. As it turns out, brownish-orange spiders called Anelosimus studiosus have apprently mastered this adaptation.

The findings were published in the journal Nature.

Charles Darwin first proposed the idea of group selection in his 1859 masterpiece On the Origin of Species. He applied the concept of natural selection, which relates to an individual, to a group setting, where species who were able to "sacrifice themselves for the common good, would be victorious over most other tribes; and this would be natural selection."

But the notion that natural selection can operate on groups, and can produce group-level adaptations, has long been a subject of debate amongst biologists.

"Biologists have never shown an adaptation in nature which is clearly attributable to group selection," co-author Charles Goodnight, at the University of Vermont, said in a statement. Until now, that is.

Anelosimus spiders may be the size of a pencil eraser, but they form organized groups that can catch prey ranging from fruit flies to small vertebrates. Not to mention that they make cobweb nests "anywhere from the size of a golf ball to the size of a Volkswagen Beetle," added co-author Jonathan Pruitt, from the University of Pittsburgh.

Two types of females live in these spider colonies that are crucial to a group's evolutionary force in nature. There are those that are "aggressive" or "docile." Normally, docile spiders dominate small colonies, and then the aggressive females take over as the group grows.

Interestingly, Goodnight and Pruitt found that in environments with limited resources, the opposite happens. Small colonies are dominated by aggressive females and the docile variety appears more frequently as the colonies grow.

"Certain ratios yield high survivorship at some sites, but not others," the researchers write.

Even when the study authors created artificial colonies and placed them in new locations, the spiders over time worked to establish a docile-to-aggressive ratio of spiders that would best benefit the group as a whole, as opposed to thinking solely of themselves.

This study is the first to demonstrate natural selection working on a collective trait that determines whether whole colonies survive or not.