Bowles, his Critics, The Price Equation, and Group Selection

Posted by Simon Halliday | Wednesday, June 24, 2009 | Category: , , , |

I have read a number of blog posts and articles on Sam Bowles's recent paper in Science, 'Did warfare among ancestral hunter-gatherers affect the evolution of human social behaviors?' Apparently, many people who are hostile to the idea for some reason (i.e. that humans, for whatever reason, might have had a violent past, or that our ancestors had behaviour patterns similar to chimpanzees) try to argue against it from poor grounds.Yet others seem to worry that 'Group Selection' differs greatly from gene selection, or kin-selection, which is not strictly accurate, as I discuss below. But not all the news is bad, some commentators make decent criticisms which need to be considered. This post is longer than normal, but I think the topic warrants such length.

One of the poorest criticisms I've seen consists of the 'Richard Dawkins in The Selfish Gene says' variety. Yes, we all know Dawkins argues against group selection models in animals. But, we also know that he doesn't mention that kin selection (à la Hamilton, 1962) is basically just a sub-class of group selection models defined by the Price equation (Price, (1970, 1972) - For a formal explanation of Hamilton's rule as a restatement of the Price equation see Rowthorn (2006)). Yes, kin selection is just group selection on a small scale. Shock! Horror! Furthermore, as Dawkins points out later in The Selfish Gene, humans are exceptional because of our cognitive abilities and because of culture. As he says, "The arguments I have put forward should, prima facie, apply to any evolved being. If a species is to be excepted, it must be for good particular reasons. Are there good reasons for supposing our own species to be unique? I believe the answer is yes" (Dawkins, 2006, 189).

With this admission we have the opportunity for gene-culture co-evolution. Genes affect our cultural expressions. Culture, because it changes the environment in which a gene finds expressions, affects our genes through evolution. Hence, having a 'cultural norm' of warfare that is culturally transmitted could affect the genetic expression of specific gene complexes, such as one that manifests altruism. Furthermore, in more recent texts such as The Ancestor's Tale, which I am currently listening to, Dawkins discusses the specific role of culture and how it could have shaped human evolution in a co-evolutionary manner.

Moving away from these issues, a commenter on one blog post evidently doesn't understand the Price equation. I reproduce it here for clarity.

$\pi \Delta p = \mbox{var}(p_j) \beta_G + \mbox{E}\{\mbox{var}(p_{ij})\}\beta_i$

Or alternatively, isolate the change in p term:

$\Delta p = \frac{\mbox{var}(p_j)\beta_{G}}{\pi} + \frac{\mbox{E}\{\mbox{var}(p_{ij})\}\beta_i}{\pi}$
Now, many people struggle to interpret this equation. It tells us (and I use the first rather than the second version) that changes in the metapopulation will reflect changes in the populations of different genetic expressions, say Altruists (As) and Non-Altruists (Ns). Now the commenter says that Dawkins asserts that it is in a gene's interests to cooperate with the other genes for a body's survival. This elides the problem of the prisoners' dilemma game in which the genes are involved. If all other genes, being As, are cooperating to ensure survival of the host, it is in the interests of any one gene, say an N, not to do anything and to free ride on the exertions of the other genes to produce a healthy and productive gene carrier. This free riding gene can express other characteristics and maximize fitness by not cooperating.

The Price equation enters here. It's also easier at this juncture to aggregate up to the human level to introduce cultural practices and to use th expression of a gene complex to make the argument. The Price equation says that you have between group pressures (the first term, which is positive) and within group pressures (the second term, which is negative). As long as the variance between groups is greater than the variance within a group, the gene will proliferate. But how do you keep the variance of the first term, or the between group differences, sufficiently high to combat the downward selection pressures of the within group dynamic? For example, it's great to have a bunch of Altruists doing their thing as long as you don't get a sufficient number of Non-Altruists who enter their group (through mutation, drift, cultural effects) and undermine them.

The commenter erred because he assumed away within group pressure. If that was the case then the problem would be trivial. Dawkins' 'Cooperative Gene' explanation does not solve the problem unless we can explain how the gene proliferates within a group given within group pressures against it.

Now, let Bowles's example of warfare enter the arena. Bowles argues that warfare was sufficient to ensure that between group variance was kept high, ensuring the success of a sufficient number of Altruists. Assume that Altruists confer benefits 'b' on those with whom they interact, and bear costs '-c' to do so. Non-altruists do nothing. Which implies that: $\beta_G = b-c$ and that $\beta_i = -c$. Using those, we can manipulate the Price equation to produce the followin:
$\frac{c}{b} = \frac{\mbox{var}(p_j)}{\mbox{E}\{\mbox{var}(p_{ij})\} + \mbox{var}(p_j)}$
This means that altruists will survive as long as the ratio of costs to benefits is sufficient to maintain the variance between the population. Moreover, when there are additional cultural norms which allow reproductive leveling, and which therefore decrease the costs to Altruism, Altruism becomes much more likely. As Bowles (2006, 1569) puts it, "Culturally transmitted norms supporting resource and information sharing, consensus decisionmaking, collective restraints on would-be aggrandizers, monogamy, and other reproductive leveling practices that reduced within-group differences in fitness may have attenuated the selective pressures to which altruists are subject." I don't believe that the blogger on the post I commented on previously understood these phenomena properly.

All of this said, John Hawks makes a decent point about resource scarcity. He argues that Bowles claims that too much falls in the ambit of warfare. But, as far as I could see, Hawks chose to ignore Bowles's hedges. The problem, moreover, is that Hawks's argument does not seem consistent with the archaeological evidence. Consider the counterfactual, if resource scarcity and population growth mediate each other to the extent that Hawks argues they do, then the fossil evidence for inter-group violence should be minimal, or, if inter-group conflict did occur, why would it necessarily be violent or widespread? The fossil evidence indicates the contrary, as does more recent anthropological evidence of hunter-gatherer groups. Now, I admit that this is not my area of expertise, but it seems apparent to me that warfare could constitute an explanation, especially in conjunction with the evidence on resource scarcity.

This brings us to a further criticism. What about Occam's Razor, is the explanation sufficiently parsimonious? Are we offering too many inter-linked factors to explain the surprising existence of sociality in humans? Let me respond as follows. Many, many explanations of cooperation have arisen with many more inter-linked factors arising to explain cooperation. But, we are engaged in an effort to try to reduce the number of explanations into separate cultural and genetic phenomena. Bowles offers one such explanation considering the environmental factors, and gene-culture co-evolution. Evolution, in this sense, is the underlying parsimonious theory with theorists trying to understand the specific parameters that need to be appropriately tooled to apprehend the eventual evolution of something as seemingly counterintuitive as - and as prevalent as - cooperation.

I hope this post adds some clarity to the debate for those of you who are interested. I also admit that I am probably quite partisan in this debate because Sam Bowles is a professor of mine at the University of Siena and, though I disagree with some of his arguments, I find those promoting the role of inter-group violence in human history quite convincing.

References:
Bowles S. 2009. Did warfare among ancestral hunter-gatherers affect the evolution of human social behaviors? Science 324:1293-1298. doi:10.1126/science.1168112
Bowles, S. 2006. Group competition, reproductive leveling, and the evolution of human altruism,
Science 314: 1569 - 1572 DOI: 10.1126/science.1134829
Dawkins, R, 2006, The Selfish Gene: 30th Anniversary Edition, Oxford University Press.
Dawkins, R, 2005, The Ancestor's Tale, Audiobook, Abridged and read by Richard Dawkins and Lalla Ward, Orion.
Price, G.R., 1970, 'Selection and Covariance', Nature 227: 520-521
Price, G.R. 1972, 'Fisher's 'Fundamental Theorem' made clear', Annals of Human Genetics, 36:129-140.
Rowthorn, R, 2006, 'The evolution of altruism between siblings: Hamilton’s rule revisited', Journal of Theoretical Biology, 24: 774–790