In mutually beneficial relationship, slowest-evolving species gains upper hand

When members of two species compete directly with each other, scientists believe the one that rolls with the evolutionary punches and adapts most quickly has the upper hand. But new evidence suggests that in relationships that benefit both species, the one that evolves more slowly has the advantage.

“The idea that has been dominant for the last couple of decades is that when two species co-evolve, they try to outrun each other,” said Carl Bergstrom, a University of Washington assistant zoology professor. But that doesn’t necessarily hold true for individuals of different species engaged in a mutualistic, or symbiotic, relationship. In such cases, he said, the one that evolves more slowly is likely to gain a disproportionate share of benefits from the relationship.

A predator, for example, typically must evolve rapidly so that it doesn’t fall behind the evolutionary advances of its prey and thus miss lunch. That has been termed the “Red Queen effect” after the character in Lewis Carroll’s “Through the Looking Glass,” who said, “It takes all the running you can do to keep in the same place.”

Bergstrom and Michael Lachmann, a postdoctoral researcher at Germany’s Max Planck Institute for Mathematics in the Sciences, have given the name “Red King effect” to mutualistic relationships in which greater benefits go to the slower-evolving species. Their findings are being published this week in the Proceedings of the National Academy of Sciences.

The effect is particularly pronounced if, in the course of “bargaining,” one side has little room to negotiate and so the other side is forced to give up more if the two are to continue their relationship, according to the researchers, who have been collaborating for nearly a decade.

For instance, one might imagine the Red King and Red Queen on opposite sides of a chessboard, with the slow-and-patient king and the fast-but-impatient queen negotiating where to meet. Each wants the other to travel farther, but the king explains that he is at a disadvantage because he can only move one square at a time. Since the queen can move as many spaces as she wants, she relents and meets her husband on his side of the board.

Bergstrom and Lachmann worked with a mathematical model devised from evolutionary game theory, which studies games in which the overall fitness of the players depends on the success each has in playing the game. The model, closely related to models used in the study of economics, can be applied to well-understood relationships between species.

In the case of ants and lycaenids, the largest butterfly family, with many different species, the ants draw considerable benefits by protecting lycaenid caterpillars from parasites that pose a great mortality threat. In return, the caterpillars spend much of their energy producing sugar- and protein-rich excretions as a food source for their ant protectors. The dilemma is how much food should the caterpillars provide and how much should the ants demand. The answer changes over time, depending on how quickly the species evolve. At times the caterpillars might offer less food and at least some of the ants might look for new food sources.

“What the rate of evolution does is tell how long one population can go on taking what’s offered,” Bergstrom said. “Every time some ants evolve and go off to look for something better, it encourages the caterpillars to offer more. But if the ants don’t leave in search of a better deal, eventually they will evolve to accept less.

“Species that evolve fast give in more to pressure than species that evolve slowly.”

Bergstrom said the Red King finding is something of an anomaly that will add to the body of knowledge in evolutionary biology.

“I don’t think natural selection is going to select for species that evolve slowly. It would be interesting and counterintuitive if that happened, but I don’t expect that to happen,” he said.

“Our work just points out one particular building block in evolutionary theory, but it will take a lot more refinement and examination before we know how important this phenomenon is in determining the form and pattern of mutualisms.”

For more information, contact Bergstrom at (206) 685-3487 or cbergst@u.washington.edu or Lachmann at (011) 49-341-995-9854 or dirk@santafe.edu