The Primate Diaries

Their results show that, in the tension between individual selfishness and limited resources, the most stable evolutionary strategy is that working together to limit growth while sustainably sharing the resources among all members of the colony results in the greatest longterm success. As the authors state in their abstract:

We present a mechanistic model of cell growth at a surface, and we show that tension between growth and competition for nutrients can explain how empirically observed patterns emerge in biofilms. We then apply our model to evolutionary simulations and observe that the maintenance of patterns requires cooperation between cells.

There have traditionally been two methods for understanding cooperation in social organizations: at the level of the individual and at the level of the system. Behavioral biologists typically study individual strategies and look at such things as kin selection (relatives helping relatives), reciprocal altruism (tit for tat) or generalized reciprocity (A helps B so B helps C). By doing so they can better understand the potential genetic template that has been selected for as the result of evolutionary pressures. Through these various strategies individuals form cooperative bonds as a way to maximize their reproductive success or that of their close relatives. Through these individual decisions higher-order social organizations emerge. This is the most common method for understanding social organization and has been remarkably successful in explaining most (though not all) examples of cooperation in the natural world (for more on these forms of cooperation see my posts here and here).

Systems biologists, on the other hand, focus on the social organization itself and seek to understand the forces that shape the pattern of a social system rather than what drives individual decisions. An example of this latter category includes the organization of insect foraging trails. As ants randomly search for food they leave pheromone trails behind them. Quite by accident, an individual ant will come across a large food source (your picnic basket for example) and will begin carrying food back to her colony. Some time later another ant will, again quite by accident, independently discover the same food source and will lay her own pheromone trail on top of her sister's. Each time another ant lays their own trail over the existing path it strengthens the signal. Like rivulets deepening into streams and later into rivers, or the repeated firing of individual synapses strengthening into neurological pathways, this accumulation of pheromone signals becomes a stronger and more entrenched path that develops into the legion of individual ants all acting in unison. As a result, random individual behaviors are transformed into a higher-order social organization.

While both biological traditions are seeking to understand the same end result, the key difference is that one studies conflict vs. cooperation between individuals while the other studies the forces that shape the emergence of social patterns. What Xavier and colleagues have now demonstrated is that both traditions are important for understanding the formation and maintenance of cooperative organizations. To illustrate this process in action they chose a subject species that few could argue has any individual choice in the decisions they make as a whole.

Bacterial biofilms are collections of individual cells that form into complex, organized patterns when placed on glass inside a liquid medium. To understand how the bacteria "choose" to cooperate, the authors developed a simple experiment utilizing Pseudomonas aeruginosa bacteria. They began by placing individual cells at random locations on a glass coverslip that together occupied only 1% of the surface. At first the bacteria behaved as expected, by forming clusters of small circular colonies around each individual cell as they reproduced. However, as the population grew and continued feeding on the available nutrients there soon came a point where there wasn't a high enough nutrient concentration to sustain their fast rate of growth. Rather than quickly eating through the available resources and dying out, the P. aeruginosa bacteria did something rather remarkable. In response to declining resources the bacteria formed into a stable labyrinthine structure, which the authors call fingering, and restricted their rate of reproduction based on the level of nutrients available. Rather than maximize their own individual growth at the expense of others, the bacteria slowed down and formed into an efficient structure that maximized the nutrient concentration for the group as a whole. Incredibly, when faced with an environmental crisis, these simple bacteria somehow have a system in place that shifts their overall strategy from selfishness to cooperation.

As the authors explained the phenomenon:

The two processes behind pattern formation can also be characterized as opposing social effects. Whereas nutrient consumption is a competitive interaction, mechanical pushing is cooperative. . . A key finding of our analysis is that the patterns in our system arise from the very same processes that were central to the evolution of cooperation and conflict in our previous models of biofilms: regional competition for nutrients and cooperative pushing of cells into nutrient-rich regions.

In other words, there are two important processes that explain these patterns in bacterial biofilms: the competition for resources and cooperation in colony growth. First, cells have a negative influence on other cells around them by consuming nearby nutrients and making it more difficult for other bacteria to find enough to eat. However, at the same time, the cells are regularly dividing and pushing each other outwards to make room for colony growth (or "mechanical pushing", as the authors call it). As a result, cells benefit by receiving a push that allows them access into an area of higher nutrient concentration. But how does this mutual expansion combined with the individual pursuit for nutrients end up forming a stable and cooperative structure?

To explain this let me return to my earlier example of ant pheromone trails. Imagine that the journey back from your picnic basket to the colony must cross over a busy sidewalk. Have you ever wondered why ant trails in the city so often fall along cracks in the pavement or in gaps between the cement? The ants certainly didn't plan this out in advance. It occurs based on the collective behavior of individual ants and the external forces that shape the outcome. When there are only a few ants following a pheromone trail the likelihood that they'll be stepped on by a random pedestrian is fairly low. But once thousands of ants begin crossing back and forth continuously it's bound to happen. When it does the pheromone trail is disturbed and the ants must adjust their path as a result. After multiple such disturbances the path is accidentally laid down inside one of these gaps where the ant trail is now protected from foot traffic. A stable system has now emerged without any one individual responsible for its development nor even understanding how or why it works.

A similar result can be found when the P. aeruginosa bacteria are all pushing each other outwards as they reproduce. The narrow passages within the emerging labyrinth are what allow nutrients to flow to the bacteria along all sides of their structure. However, when the collective mechanical push causes one of the labyrinth passages to narrow, the cells on the inside can't get enough nutrients to reproduce and they slow their growth. This causes the labyrinth passage to expand once again as the cells die off naturally, allowing equilibrium to return. As the colonies continue to expand outwards their cooperative structure remains intact, ebbing and flowing along the gradient of nutrient concentration and their reproductive ouput. As remarkable as it may seem, complex cooperative systems are the end result of nothing more than the collective behavior of individuals and the external forces that shape their outcome. The ideology of the cancer cell has no place in a system that seeks to maximize survival over the long term.

Edward Abbey's concern that humans are behaving in an erratic and self-destructive manner may be an accurate representation of our current rate of growth. However, if the microbial world is any guide, we are by no means doomed to metastasize and bring the entire ecosystem crashing down along with us. In fact, sociologists who have looked at international conflicts over shared water resources have found that cooperation is the norm rather than the exception. When faced with the loss of such an essential resource, the emergent behavior seems to be a stable structure based on mutual benefit. It's possible that the human species is now approaching a level of global saturation that the P. aeruginosa bacteria were forced to respond to in their microhabitats. Will we respond the same way? If so, the organizational structure that we may ultimately achieve is still in its early stage of formation. Finding a sustainable way to live that conserves our collective resources while at the same time maintains our individual needs will be the ultimate task of this and all future generations. While the prospects are certainly dire it's somewhat reassuring to know that, while our individual actions will continue to be important, there may be natural systems in place to help shape the outcome whether we're aware of it happening or not.

Reference:

Xavier, J., Martinez‐Garcia, E., & Foster, K. (2009). Social Evolution of Spatial Patterns in Bacterial Biofilms: When Conflict Drives Disorder The American Naturalist, 174 (1), 1-12 DOI: 10.1086/599297