At its core, evolution is about pressure. Previously, researchers thought that organisms in mutualist relationships—in which two or more organisms benefit one another—would be quite comfortable and therefore evolve slower than others. But new research published in the journal Nature Communications finds the opposite might be true.   

Ants and their plant buddies are some of the most successful mutualists on Earth. The plants provide a safe place for the ants to live, while the ants keep the plants safe from herbivorous insects. 

Ants’ successful setups don’t always catch on with their relatives. The genus Pseudomyrmex comprises three families of ants: one group of mutualists that make their homes in the enemy-deterring thorns of acacia trees, and two non-mutualist groups that live on their own in related plants.

Mutualist ants on acacia thorns. Image credit: Alexandra Westrich, The Field Museum

The difference in their lifestyles is apparent in their behavior. Mutualist ants are so confident in their safety that they leave the nest and form patrols in order to hunt down plant-eaters and other invaders. Generalists, on the other hand, seem to live in terror, fleeing from any aggressor, even when that means abandoning their nests. 

Biologists were curious to see how some ants became mutualists while their cousins did not. They analyzed the genomes of seven Pseudomyrmex species—three mutualists and four generalists. Because the generalist species live in such stressful conditions, the scientists expected that they would have evolved more rapidly than their gutsy, cushy-living cousins. But the reverse seemed true. The three mutualist species had evolved much faster than their generalist kin. 

Why would anyone in a happy relationship—ant, plant, or otherwise—keep changing? The scientists aren’t totally sure, but they have a theory. 

"Just like all organisms, mutualists need to constantly adapt to their environments to ensure survival,” says co-author and Field Museum curator Carrie Moreau. “However, they also have the additional task of evolving in relation to one another. Many species flip between mutualism and parasitism over time. Even mutualism is a costly relationship that evolution will select against if it is no longer advantageous. All of this likely factors into the accelerated rate of evolution among these species."

Let this be a reminder to us all: Even the best relationships need work.

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