Sunday, June 7, 2015

Bacteria Form Communities Too

Moral issues among the tiniest life forms.

Sometimes it takes a village, even for bacteria. Famous for being mindless, pitiless, evolutionarily-honed automatons, it turns out that bacteria have need of community and have ways to detect and signal that a community is ready for action. Not only that, they have ways to punish cheaters who do not pull their weight for the collective, tiny as it might be.

A recent computational biology paper developed game theory models based on bacterial dynamics to explore what signals are needed for group cooperation and how they evolve. Suppose that some bacteria face a food source (say a piece of wood) that requires acidic pH to digest. One bacterium isn't going to make much headway by itself. But if a thousand gather and crank out some protons in unison, they could make themselves a feast. Then what if one of them decides to loaf around, eating its fill but not producing any acid to help digestion?

This presents a series of problems. First, each bacterium needs to be able to tell whether there are enough colleagues about to make a collective / individual effort worthwhile. Second, it needs to know whether such a decision is shared by the others, so that they act in unison. And third, of course, they need to devise some way to punish or mitigate cheating and loafing.

Incidentally, these authors refer routinely to a personified "Nature", as in "We shall assume that Nature may choose between two different states ...". This is somewhat off-putting, and doubtless a consequence of being computationally oriented, making them wish to make it clear when they are speaking of natural cases, versus in silico, modelled cases. This should, however, not create any presumption of a theology of some shadowy entity behind the curtain or pan-consciousness, etc.

The authors simulated basic game theory conditions where individuals either signal or do not signal, with varying costs, and either cooperate or do not, responding or not responding to the majority signal around them. The idea was to vary the costs and benefits, and introduce mutants that employ other strategies, to ask what conditions lead to stable conditions, especially those resembling the world we actually see, where cooperation is, among bacteria, reasonably common.

In fairness, they asked themselves quite limited questions, focussing on whether signalling systems routinely evolve under conditions where cooperation is beneficial. Obviously, the answer to this is going to be yes, under any state where the conditions vary and are not adverse, requiring cooperation, all the time. The strategies they entertained were brutally simple- either signal or don't, and either cooperate or don't. So there was little subtlety.

Bright / beige areas show where most of the simulated populations reside under various conditions, while practicing the cooperative, signalling responsive strategy. The Y axis is cost of signalling. Very high costs (as a proportion of the benefit gained) do inhibit signalling, but this is biologically not very realistic. The various graphs then show: 1. selection pressure for the cooperative benefit, which strongly stablizes cooperation, as well as signaling  2. As the cost of cooperation rises, populations are OK with higher signalling costs, given a relatively high selective constraint for cooperation, (gamma 𝛾 is set at 5). 3. If the rate of bad conditions rises to high levels, it makes no sense to signal that fact, really, and populations cooperate, either without signaling at all (i.e. all the time) or in response to a lack of signal.

One further problem is that the authors have given themselves the simplification that only one mutant form can exist at a time, either invading and taking over the whole population, or dying out before the next mutant strategy comes onto the scene. That means that a shirker strategy of benefitting from the cooperation of others may briefly invade a more cooperative population, but can never realistically take over the population when the benefits of cooperation (i.e. selective pressure for cooperation) are set high.

Thus the main question that we would realistically have about cooperative strategies, which is about the stable rate of shirking in otherwise cooperative populations, and the evolution of counter-strategies of detection and punishment, never come up in this analysis.

Thankfully, there is other work in the field that has established punishment strategies, though not yet any active surveillance and detection strategies, which might be beyond bacteria's cognitive capacity. In one example, the human pathogen Pseudomonal aeruginosa secretes small amount of cyanide, which the cooperators are resistent to, but the cheaters are not.

Small as they are, bacteria face basic dilemmas of survival and group action. They have the glimmerings of moral needs and one can readily see how increased capabilities led to increasingly complex behaviors of evasion, deception, and group control that we express in our moral systems.

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