Spiders May Help Prove "Group Selection"

Issue Date: 
October 20, 2014

The notion of “group selection”—that members of social species exhibit individual behavioral traits that render a population more or less fit for survival—has been bandied about in evolutionary biology since Darwin. The essence of the argument against the theory is that it’s a “fuzzy” concept without the precision of gene-based selection.

Jonathan Pruitt, assistant professor of behavioral ecology in Pitt’s Department of Biological Sciences in the Kenneth P. Dietrich School of Arts and Sciences, published a paper Oct. 1 in the leading journal Nature that may be proof of group selection’s validity. 

Other studies have hinted at the legitimacy of group selection. But, Pruitt says, “Our study shows group selection acting in a natural setting—on a trait known to be heritable—and that has led to colony-level adaptation.” 

Pruitt, and colleague Charles Goodnight of the University of Vermont, examined colonies of Anelosimus studiosus spiders composed of a mixture of individuals with docile and aggressive traits. In nature, individual colonies differ in their docile-to-aggressive ratio.

Pruitt constructed experimental colonies of known composition, tweaking the docile-to-aggressive ratio in each. What he found was that docile-to-aggressive ratios are driven by the site at which the colonies are placed. Certain ratios yield high colony survivorship in certain geographical locations but not in others. 

Over time, the span of two generations, the docile-to-aggressive ratios of perturbed native colonies—those that have lost their site-specific optimal docile-to-aggressive ratio—change. Colonies that find themselves at risk of extinction reestablish their group composition to better match the ratio that their native site calls for. The experimental colonies that were moved across sites, however, changed their ratio to one that “would have promoted their survival had they remained at their home site, regardless of their contemporary environment,” Pruitt says.

“These findings provide compelling evidence that the mechanisms colonies use to regulate their compositions are themselves locally adapted, presumably because of the survival advantages they confer to the colony,” Pruitt says.