cooperation

Sharon Begley in Newsweek reports on a hypothesis about "collectivism" and pathogens:

The West epitomizes individualistic, do-your-own thing cultures, ones where the rights of the individual equal and often trump those of the group and where differences are valued. East Asian societies exalt the larger society: behavior is constrained by social roles, conformity is prized, outsiders shunned. "The individualist-collectivist split is one of the most powerful differences among cultures," says Nisbett. But the reason a society falls where it does on the individualism-collectivism spectrum has been pretty much a mystery. Now a team of researchers has come up with a surprising explanation: disease-causing microbes. Societies that evolved in places with an abundance of pathogens, they argue, had to adopt behaviors that add up to collectivism, for reasons of sheer preservation. Societies that arose in places with fewer pathogens had the luxury of individualism, which is less effective at limiting the spread of disease but brings with it other social benefits, such as innovation.

There have been a lot of papers lately trying to match clines (that is, gradients) of phenotypes or genes with current ecological conditions. Climate is the most frequent (often, measured in terms of temperature or rainfall). Pathogens sort of follow the climate gradient, with some exceptions -- sometimes but not always allowing for the fact that malaria is the largest.

But in some cases, much more important will be the historical dimension. Many, (but not all) people who live in high-pathogen areas today had ancestors who adopted agriculture relatively early, began living in denser concentrations and larger groups, and who therefore experienced in common a range of selective pressures that have nothing to do with pathogens.

Would it be surprising that early agriculturalists living in emerging villages and cities might have been subject to pressures that enhanced collectivism? Such changes may have been facilitated by genetic changes, but would have also included cultural adaptations. Yet a correlation with pathogens would emerge as a side-effect of the history of agriculturalism, not as a direct cause.

To my mind, these kinds of historical correlations rooted in ecological change will be a central problem of anthropological genomics. Recent human evolution has been dominated by a few very large changes -- like boulders thrown into a pond that have spread massive ripples through many elements of human genetics, anatomy, and behavior. These changes are not yet complete -- the ripples have not settled down nor reached the shore. For this reason, there will be many correlations that have been induced by the large ecological changes, making bivariate spatial comparisons a poor test of cause.

I would say that historical correlation is a problem with a number of recent studies of cranial variation. Lately, it has been fashionable to test the hypothesis that cranial variation is adaptive to climate, by looking for spatial correlations between cranial measurements and climatic variables. But this test assumes that the correlations have not emerged as a result of some other cause. That might not be such a bad assumption, if humans had been static within their current geographic range for a long, long time. But humans have been anything but static -- in fact, their dynamism over the last 40,000 years has been the cause of profound changes in human biology. Naturally, things will be correlated with climate, because climate has been correlated with human subsistence and population size changes.

A better test of adaptation on cranial variables would propose concrete mechanical (or developmental) reasons why a cranial trait helps someone to better survive under a given climatic regime. Finding a correlation in space is not enough. This is why Bergmann's and Allen's rules are more compelling than the more nebulous idea that "facial form" adapts people to their climate.

Likewise in the case of pathogens -- a correlation between current pathogen load and current behavioral "collectivism" does not suggest a causal relationship between the two. Instead, we would want some kind of functional hypothesis to account for pathogens causing people to change in their attitudes toward cooperating. Concepts like the "luxury of individualism" make little sense. Of course, some people will prefer to behave with individual autonomy. But how would a recurrent epidemic disease stop them?

Burtsev and Turchin (2006) present the results of simulations of cooperative behavior in self-interested agents. This is a well-established subject, and their contribution is that their strategies are "evolved" from basic behavioral elements within their simulations, instead of being assumed a priori.

In our model, agents are endowed with a limited set of receptors, a set of elementary actions and a neural net in between. Behavioural strategies are not predetermined; instead, the process of evolution constructs and reconstructs them from elementary actions. Two new strategies of cooperative attack and defence emerge in simulations, as well as the well-known dove, hawk and bourgeois strategies. Our results indicate that cooperative strategies can evolve even under such minimalist assumptions, provided that agents are capable of perceiving heritable external markers of other agents.

This to me is one of the most interesting aspects of the model: behavioral traits gain random associations with recognizable phenotypes, and individuals shape their behavior according to the phenotypes that they detect around them.

Each agent has external phenotype that is coded by a vector of integer values (markers). The markers do not influence behaviour but function only as indicators of similarity....All of our simulations were started with an initial population of agents that were unaware of markers (the matrix coefficients connecting input from markers to actions were preset to zero). Thus, the use of markers in a population had to evolve from a blank slate. Because markers and behaviours are not linked (apart from both being inherited from the ancestors), agents can lose cooperative behaviours by mutation while retaining 'in-group' markers. Thus, the structure of the model allows free-riders to arise.

This "phenotypic association" vector is suggestive. Of course, for real animals it would probably be more effective to recognize the behaviors themselves as signs. But this depends on multiple opportunities -- you have to see somebody else's behavior at least once to judge it. If there were external manifestations associated with behaviors, it would give the opportunity to decide before an interaction what the other individual's likely strategy would be.

But then, selection would favor mimicry -- free-riders with the phenotypes of cooperators, for example. This force will tend to limit the degree of association between observable traits and behaviors...

That observable traits that indicate relatedness will also tend to indicate similarity in cooperator phenotypes. In other words, you can figure that your relatives will tend to act like you, and also tend to look like you. And as a bonus, if you are sharing with a relative, you are increasing your inclusive fitness.

They find that the evolution of different strategies depends on carrying capacity, and some new strategies emerged. The comparison of these is worth reading, but a bit too long and involved to quote at length. This part is important:

Our results have important implications for the evolution of territoriality in animals (and private property in humans). With a few exceptions, theorists have paid little attention to the role that cooperation may have in the evolution of territoriality. Our study suggests that cooperative defence of territory can radically change the course of evolution in resource-rich (C > C2) environments. When the amount of resource becomes large enough to support more than one agent, and too large for a single agent to monopolize, solitary bourgeois are replaced by cooperative starlings, provided that agents can recognize in-group members. The starling strategy does not take over completely, however, but coexists with other strategies in a complex dynamical way.

The "starling" strategy is a mobbing strategy, in which small animals cooperate to attack and drive away large solitary predators.

One limitation of these kinds of simulations is that they don't include reproductive boundaries. For this reason, they don't really distinguish models of within-species cooperation from between-species mutualism. Different strategies like "hawk" and "dove" might really represent predators and prey species, or they might represent contrasts of competitive behavior within a species.

So the appearance of stable strategies at any given level of possible complexity might be a constraint on natural communities, but the level of that constraint may not be immediately obvious. To that end, this study has a very large possible set of strategies (more than 10¹⁰⁰⁰ combinations), which means it is sampling a richer set of behaviors than most simple game-theoretic models.

References:

Burtsev M, Turchin P. 2006. Evolution of cooperative strategies from first principles. Nature 440:1041-1044. DOI link