A queen ant chews through the thorns of the acacia plant. Once inside, she finds shelter to lay her eggs. Outside the thorn, more ants smell rich nectar dripping from the leaves, and after tasting the sweet sap, they are hooked. Soon, the plant houses an entire colony of ants, providing them with a nesting site and food. In return, the ants act as the plant’s bodyguards, aggressively fending off herbivores with their painful stings. These two species are in a mutualistic relationship—a partnership where each organism benefits from the other’s activities—and life seems too good to change.
And maybe life doesn’t change—at least, not rapidly. According to a scientific theory called the Red King effect, organisms in mutualistic relationships evolve slowly to maintain their beneficial relationship. The theory is an offshoot of the Red Queen Hypothesis, which proposes that organisms must constantly evolve to survive because of interspecies competition and predation. For example, if a rabbit population evolves to better escape foxes, then the foxes must adapt to better catch the rabbit or go extinct from starvation. The selective pressures on both species lead to faster evolutionary rates.
While the Red Queen Hypothesis explains predator-prey relationships, the Red King effect relates specifically to mutualism. “When two organisms are working together rather than fighting each other, than maybe they should be evolving more slower, so as not to outrun each other,” said Benjamin Rubin [2], lead author of a study at the University of Chicago on the subject. Rubin studied a mutualistic ant-plant relationship, observing ants that take shelter in the hollow thorns, trunks, or leafstalks of plants while aggressively patrolling and protecting against herbivores. According to the Red King effect, Rubin should have witnessed a slower rate of evolution, but he did not.
Rubin’s team did not originally plan to study the Red King effect. They were comparing the DNA of several closely-related mutualist and non-mutualist ant species, when, without even looking for it, they detected a pattern: the mutualists evolved faster. “Just trying to explain that pattern, that phenomenon, led me to explore the Red King effect,” Rubin said.
To explore this phenomenon, Rubin’s team used a field of biological research called comparative genomics, where they compared the DNA sequences of different species. Genetic mutations drive evolution by creating the heritable variation necessary for natural selection. Therefore, by observing the number of accumulated sequence changes, or mutations, since a common ancestor, researchers can predict evolutionary rates.
This type of research would not have been possible 10 years ago. “Each genome used to cost millions of dollars and years of effort, but with recent advances in technology, we are now able to sequence full genomes relatively easily,” said Rubin. “It’s all done on the computer and through programming. In addition to needing the sequencing technology, we also need certain computing technology in order to do any of this.”
Rubin’s discovery directly contradicts the Red King effect. However, there is not enough information to prove whether mutualism accelerates or slows evolution, since the underlying causes are still unknown. Rubin plans to explore potential mechanisms in future research, but in the meantime, he has his theories. For example, the biological interaction between plants and ants might increase selection pressures: By being in an intimate, mutualistic relationship, each species must adapt not only to the selective pressures on them, but with the organisms they closely interact with as well. A dietary effect may also contribute. In this mutualistic relationship, ants rely on the food resources provided by the plants. The nectar contains an enzyme called chitinase that inhibits one of the ant’s digestive proteins, preventing them from breaking down other sources of sucrose [4]. Due to their reliance on this nectar, the ants must adapt to accommodate changes in the chemical makeup of their food. Consequentially, these mutualistic ants seem to accumulate more mutations and evolve faster.
By challenging the Red King effect, this study has brought attention to evolutionary relationships and opened the potential for further research. Rubin plans to continue studying the effects of behavior on the genome. While the biology behind the differences observed in mutualist and non-mutualist ant species is uncertain, one thing is for sure in this co-evolutionary race: it pays off to help others.