Cooperation and Experimental Evolution

Cooperation and altruism are widespread in biology, from molecules and genes working together in a cell, to bacterial communities that require coordinated behavior to survive in a tough environment, to human relationships and societies. Our human cultural perspective (perhaps even more specifically our American cultural perspective, focused as it is on individuality, free markets, and the American Dream), however, treats cooperation as an outright anomaly that has to be explained away by science (or often, religion). If natural selection is about the "survival of the fittest" how can a selfless gene be rewarded evolutionarily, surviving to the next generation? If evolution is about individuals locked in a battle for resources, why would anyone share with a friend?

Many experiments have shown that cooperation may actually not be so anomalous, and in fact may be a driving force for evolutionary change and diversity. Using experimental evolution, a synthetic approach to evolutionary theory where researchers try to observe evolutionary changes in controlled populations in the lab, several groups have shown that symbiotic, cooperative, and altruistic behaviors can rapidly evolve in many different situations. A recent paper from PLoS Biology, "Experimental Evolution of a Plant Pathogen into a Legume Symbiont" (ht Coturnix!) applied this kind of synthetic approach to plant/bacteria cooperation (in depth paper synopsis here). Many plants and bacteria have evolved a complex mutualistic relationship, where the plants will protect the bacteria from the harsh soil environment and the bacteria will provide crucial nutrients to the plant. Rhizobia are species of bacteria that invade the root tissue of legume plants and form small nodules where the bacteria grow and provide nitrogen that the host plant needs to grow. These species have coevolved to this complex mutualistic relationship over millions of years, but the authors found that after only a few generations pathogenic bacteria developed many of the behaviors required for the root nodule symbiosis, with many implications for evolutionary theory. From the paper's conclusion:

Our results show that adaptive genomic changes indeed allow effective dissemination of symbiotic traits over large phylogenetic and ecological distances. The fact that a single gene played a major role in the shift from extracellular pathogenesis to endosymbiosis reinforces previous reports that global regulators are preferred targets for evolution and supports fluid boundaries between parasitism and mutualism.

Other researchers have found similar results in very different model systems. Myxobacteria are single-celled organisms that live in large populations. When food is scarce, the bacteria activate a complex cascade of events where the population transforms itself from slime to a complicated fruiting body, with individuals performing highly specialized behaviors, including a huge number of individuals sacrificing themselves to provide food for the remaining cells, the ultimate altruistic behavior. Genetic mutations in a population of myxobacteria will lead to "cheater" cells that won't go through the same changes when they are on their own and starve, but will free-ride on altruistic neighbors in a mixed population with the wild type strain. A 2006 paper in Nature, "Evolution of an Obligate Social Cheater to a Superior Cooperator", started with this "obligate cheater strain" and allowed it to compete against the wild type in a laboratory evolution setup. They found that a single mutation in a gene that had previously not been identified as important for this process was able to turn the cheater strain not only into a cooperator, but into a cooperator that was able to outcompete all the ancestral strains. Scientists don't fully understand the processes that underly many of these cooperative interactions, but what is clear is that the evolution of these behaviors seems to be faster and more likely to spontaneously emerge than many people think.

There is a huge diversity of cooperative relationships in nature that expand the ability of living things to inhabit all ecological niches, but symbiosis has an even more central role in the evolution of life on earth. It is now widely accepted that complex eukaryotic cells (cells with a nucleus, like our own) evolved as the result of symbiosis between different prokaryotic (no nucleus, like bacteria). Organelles inside eukaryotic cells that provide energy, like chloroplasts in photosynthetic organisms and mitochondria in almost every eukaryotic cell, often have their own genetic material, left over from the time when they were free-living organisms. When Lynn Margulis proposed this serial endosymbiotic theory of eukaryotic evolution in the 1960's (based on theories of Russian botanists in the late 1800's and early 1900's) she was ridiculed by the biological establishment. The dogma at the time held not only that evolution must proceed through the accumulation of only small incremental changes, but also that symbiosis was a weird thing that only a few species of fungi did, not important stuff to molecular biologists. To the scientists, the notion that nature was cruel, that cooperation shouldn't exist was the norm, and the idea that evolution could be driven by cooperation at such a large scale was unthinkable. Margulis persevered, and eventually everyone realized that she was right all along. Decades later however, the marginal, "weird" status of cooperation in the biological literature remains, and Margulis warns that we should be careful with the words we use when discussing it, that we shouldn't color our understanding of reciprocal biological relationships with how we think about economics, politics, and other human constructions. Other synthetic biology experiments will perhaps further show that cooperation may be more "natural" than science currently allows it to be.