Saturday, June 29, 2013
Group Selection and the Evolution of Morality
Perhaps the best single article I have read on the group selection debate is “Evolution ‘for the Good of the Group’”, in American Scientist, September-October 2008. It is another collaboration between the two Wilsons, E. O. and D.S. I would add that it is also a very good introduction to the general question of the levels of selection‑genes, individuals, and groups, within a population.
Group selection is one explanation for the evolution of altruism. Any time one organism (or any unit within an organism) behaves in such a way as to confer a reproductive advantage to another organism at its own expense, this is evolutionary altruism. Honey bee workers who serve the queen but do not themselves reproduce are behaving altruistically. A vampire bat who regurgitates some hard won blood to feed a hungry roost mate is another example. Many examples of altruism are easily explained in terms of deferred gratification (reciprocity) or benefit to closely related individuals (kin selection).
Group selection theory is based on the claim that some altruistic behaviors are selected for because they benefit the group without any return to the altruist whether direct (deferred gratification) or indirect (kin selection). A group with more altruists will be more reproductively successful than a group with fewer and so altruists may increase in the total population, at least initially. Increase in the total population is what we mean by evolution.
There seems to be an insuperable problem. While between group selection might well favor altruistically endowed groups, within group selection will favor the selfish over the public spirited organisms. Altruists would seem to be doomed to inevitable extinction as their selfish fellows outbreed them. In this view, which held the field for a long time, group selection is unsustainable.
However, group selection does in fact occur. Wilson and Wilson present a number of forceful examples. My favorite is the bacterium Pseudomonas fluorescens.
When this species is cultured in an unstirred broth, the cells soon consume most of the oxygen in the bulk of the medium, so only a thin layer near the surface remains habitable. A spontaneous mutation called wrinkly spreader causes cells to secrete a cellulosic polymer that forms a mat and helps them colonize the water surface. Production of the polymer is metabolically expensive, which means that nonproducing “cheaters” have the highest relative fitness within the mat; they get the benefit of the mat without contributing to its upkeep. However, if the proportion of cheaters grows too high, they are undone by their own success. The mat disintegrates, and the entire group sinks into the anoxic broth. Experiments by Paul B. Rainey and Katrina Rainey have shown that the wrinkly spreader trait is maintained in the population by group selection, even though it is disadvantageous within any one group.
This example illustrates the fact that the “free rider problem” is real. The benefits of altruism in between group selection can indeed be undone when selfish cheaters crowed out the altruists. At the same time, the very fact that mats form at all demonstrates that group selection was a powerful force in the evolution of this microbe. Wrinkly spreader can only be maintained by its benefit to the community.
Obviously, what is needed to maintain group selection is some mechanism for suppressing cheating. I have no idea how this is done by bacteria but Christopher Boehm has a good idea how it is done among human hunter gatherers. He argues in Moral Origins that social selection (reproductive benefits that result from a reputation for altruistic behavior) and sanctions against bullies (free riders) functioned to protect altruists from cheaters.
Human beings are extraordinarily capable of altruism toward unrelated individuals. Explaining this is a big challenge for evolutionary theory. Boehm considers a number of explanations that are current in the scholarship. He doesn’t reject them, but argues that some of them work only when cheating is suppressed by the mechanisms mentioned above.
In Plato’s Gorgias, Socrates argues that justice is analogous to medicine: it is a response to dysfunction in the social body. I am inclined to think that the theory of group selection is beginning to uncover something like the Platonic idea of justice. It may be that retribution is something that shapes all life on earth.