Antisocial evolution

A reader writes:

ME, this article made me think of you. It reminded me of some of the discussions I've had with your readers, regarding the perceived evolutionary advantage of being a sociopath. Here, we have a colony of traditionally cooperative, successful microbes, placed in an environment with unprecedented access to resources, resulting in the emergence of genetically-induced anti-social behavior. Obviously, the scales are different, but the parallels between this experiment and post-tribal human society, as it relates to the emergence of sociopaths, are certainly striking. Of note is the observation that anti-social behavior is only advantageous so long as socialization is still the norm, and once the majority of the population stops enforcing social behavior, the entire colony dies under stress. The implication that anti-social behavior only appears advantageous in the short term, being a complete evolutionary dead-end should the genes become dominant, is a topic I thought you might enjoy dissecting. Enjoy.

“It just makes you ask, ‘What on earth is that doing?’ ” said Gregory J. Velicer, a former student of Dr. Lenski’s who is now an associate professor at Indiana University. Dr. Velicer experienced this bafflement firsthand while watching the evolution of a predatory microbe called Myxococcus xanthus. Myxococcus swarms lash their tails together and hunt in a pack, releasing enzymes to kill their prey and feasting on the remains. If the bacteria starve, they come together to form a mound of spores. It is a cooperative effort. Only a few percent of the bacteria end up forming spores, while the rest face almost certain death.

This social behavior costs Myxococcus energy that it could otherwise use to grow, Dr. Velicer discovered. He and his colleagues allowed the bacteria to evolve for 1,000 generations in a rich broth. Most of the lines of bacteria lost the ability to swarm or form spores, or both.

Dr. Velicer discovered that some of the newly evolved bacteria were not just asocial — they were positively antisocial. These mutant cheaters could no longer make mounds of spores on their own. But if they were mixed with ordinary Myxococcus, they could make spores. In fact, they were 10 times as likely to form a spore as normal microbes.

Dr. Velicer set up a new experiment in which the bacteria alternated between a rich broth and a dish with no food. Over the generations, the cheaters became more common because of their advantage at making spores. But if the cheaters became too common, the entire population died out, because there were not enough ordinary Myxococcus left to make the spore mounds in the times of famine.

During this experiment, one of Dr. Velicer’s colleagues, Francesca Fiegna of the Max Planck Institute for Developmental Biology, discovered something strange. She had just transferred a population of cheaters to a dish, expecting them to die out. But the cheaters were making seven times as many spores as their normal ancestors. “It just made no sense,” Dr. Velicer said. “I asked her I don’t know how many times, ‘Are you sure you marked the plates correctly?’ ”

She had. It turned out that a single Myxococcus cheater had mutated into a cooperator. In fact, it had evolved into a cooperator far superior to its cooperative ancestors. Dr. Velicer and his colleagues sequenced the genome of the new cooperator and discovered a single mutation. The new mutation did not simply reverse the mutation that had originally turned the microbe’s ancestors into cheaters. Instead, it struck a new part of the genome.