The controversial theory of "group selection" -- that whether a social species is more fit or less fit for survival can be influenced by behavioral traits of individuals within it -- may be valid, a new study suggests.
A notion that has been hotly debated in the field of evolutionary biology since the time of Darwin, the theory holds that a mixed group of individuals of a species can be "selected" as a group for survival and thrive over other heterogeneous groups from the same species.
As a theory it was pretty much discounted by the 1960s, as genetics-based selection rose to the fore, but the old idea has been making a slow comeback among some evolutionary theorists, who have given it a new name: multilevel selection theory.
In a new study, behavioral ecology professor Jonathan Pruitt of the University of Pittsburgh and colleagues studied a species of spider, Anelosimus studiosis, which can show either aggressive or docile traits, and whose colonies in nature can differ in their aggressive-to-docile population ratios.
Pruitt and his colleague Charles Goodnight of the University of Vermont created artificial colonies in which they could tweak the docile-to-aggressive ratio to see what the effect might be on colony survival.
The fundamental idea of group selection is that in any social species where an individual's survival is tightly bound to the performance of its group, evolution will favor traits that help increase the group's success.
This, in turn, raises the long-term evolutionary success of each individual.
Pruitt and Goodnight found that their social spiders not only work cooperatively in groups; they also evolve together as a group.
What they saw is that a shifting ratio between aggressive spiders and docile ones -- a collective characteristic of an entire colony -- would determine whether a colony could adapt to specific differing locations.
"Certain ratios yield high survivorship at some sites, but not others," the study, published in Nature, notes.
Natural colonies that were perturbed and moved away from a location where they had optimized a beneficial docile-to-aggressive ratio were able to quickly sense their survival was threatened and adjust their ratio to suit the new environment.
However, when the researchers moved their artificially tweaked colonies, the spiders tried to recreate the ratio that worked in conditions where the colony first evolved, even though the conditions in the new environment were significantly different.
"They're continuing to make the phenotypes, the trait at a group level, that would have been advantageous if they had stayed home," Pruitt says, "But they seem to have no idea that they're at a new site and that what they're doing is going to doom the whole colony."
Their experimental findings, they say, suggest natural selection focused on a collective trait -- the aggressive-to-docile ratio, which can pass down from generation to generation -- can lead to an adaptation that controls whether entire colonies survive or die.
"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 says.
In other words, the researchers say, group selection has been observed in the wild.