Sunday, May 21, 2017
Kin Selection as Group Selection Updated
In the paper I presented in Vancouver, there were three new ideas. They were new at least to me! One is that kin selection is in fact group selection: the target of selection in cases of inclusive fitness (I risk my life to save three brothers) is clearly not me. It is instead the genetic inheritance of my family.
The second is that in primate groups and especially in our hunter-gather ancestral groups a very sophisticated form of reciprocal altruism is presenting itself. When the group policies the behavior of the individual members, a social contract is in force. The parties in this case are not two individuals, as in one animal doing a favor for another when it is likely that the other will return the favor. A group that protects its members by sanctioning bullies and free riders is enforcing a social contract between the individual and the group itself.
The third idea is that the once the group protects each individual within it, one of the things that can be protected is the access of each individual male to his mate. This is, effectively, the social construction of marriage. It seems likely that the nuclear family as we understand it is a product of the emergent social contract. This is vital because, once it happens, the group can recognize not only the bonds between couples within the group, but bonds that extend beyond the group to the wife’s family.
Here I discuss the first idea. Early in Human Evolutionary Psychology (Louise Barrett, Robin Dunbar, & John Lycett) group selection is briefly considered and summarily rejected. It is only the fitness of individuals that counts in natural selection. Almost immediately the text complicates the matter by considering the selfish gene argument. I will argue that it is complicated in the other direction as well.
These are the basic models for the evolution of cooperation. Kin selection (or inclusive fitness) indicates cooperative behavior that is selected for because it benefits closely related individuals. In the paradigmatic example, if I call out because I see a tiger and so save the lives of five brothers, I may get more of the genes that code for my familial sacrifice into the next generation than if I survive and sire children of my own.
Reciprocal altruism indicates a tradeoff: one individual pays a cost in order to benefit another because it is likely that the cost will be returned. When one vampire bat shares blood with another because the other be expected to reciprocate later, that benefits both parties.
Group selection indicates cooperation on the part of a member of the group that benefits the group but not necessarily the altruist. Consider a group of organisms bound together and living in a pond. When they run out of oxygen, the group must rise to the surface. Producing the substance that allows them to rise requires energy and thus involves a sacrifice. The sacrifice only pays off if enough individuals make the sacrifice to get the group where it needs to go.
What occurred to me as I was writing this paper is that in all these cases, the target of selection was more than one individual. The term “target of selection” indicates the actor whose behavior or trait was selected for. If a well-camouflaged insect avoids predators and so successfully mates and reproduces, we can say that individual was the target of selection.
In cases of group selection, the group is by definition the target of selection. It is the fitness of the group that is favored by natural selection; individual members benefit only in so far as they are along for the ride.
The same is true, however, for reciprocal altruism. Consider the case of cleaner fish and their predatory clients. The former consumes parasites off the skin and jaws of the latter. Each must give up something: the cleaners eat only the parasites and not the healthy tissue; the predators refrain from eating the cleaners. What is the target of selection in this case? I submit that it is the alliance. Together they are selected for; divided they are selected against.
In the case of kin selection, the target is not the individual who calls out the warning, for she is more likely than the others to attract the predator’s gaze. Nor is the target of selection any individual sibling or cousin. According to Hamilton’s rule, cooperation among kin can be selected for when c < rb. Here c is the cost of the cooperative act, measured in the probability of successful reproduction on the part of the cooperator. If a certain action is likely to result in reproductive failure one out of four times you do it, then c = .25. If reproductive failure is almost certain (I charge the lion to save my brothers and sisters) c = close to one. How closely I am related to the individuals I am taking a risk for is r. Siblings are .5. How many siblings I am saving is the benefit, or b.
This explains the paradigmatic example deployed above. If I sacrifice my life to save three brothers, then 1 < .5 x 3. What is key here is that the target of selection, the T whose reproductive success is being selected for, is not any individual. The target of selection is the family.
Two things stand out for me. One is that kin selection seems to provide a very limited selection pressure in organisms like ourselves (as opposed to social wasps, bees, and ants). For siblings, r = .5; but for cousins it is only 12.5. Only if the clan were a very strong institution, so that such a sacrificial act could reliably benefit a large number of relatives, would it be able to work.
The other is that it runs into some of the same problems that group selection models face. In the latter, the altruists in the group enable the group to grow larger due to their sacrifices; however, non-altruists in the group benefit more since they are making no sacrifices. Eventually, the cheaters will outnumber the altruists and the system will collapse. This is why many theorists suppose that group selection cannot work. To make it work, you need mechanisms that enable altruists to find each other and to exclude cheaters.
Kin selection is supposed to be more reliable because I can be more certain who my real siblings are and so my sacrifice will promote the genes that code for my behavior. How reliable is this? Only once the family is institutionalized can I have a good idea who my brothers and sisters are. If the nuclear family emerged first in the course of human social evolution, and human beings were more rather than less monogamous before they joined together in larger units, then kin selection might have been the original driving force in that social evolution.
If, however, the family is the product rather than the cause of our evolutionary trajectory towards political animals, then kin selection is a much less significant (which is to say, not insignificant) factor in that trajectory. Aristotle was surely correct to say that human beings are more a coupling than a political animal, since that is true of all sexually reproducing vertebrates. Coupling, however, does not mean “familial”. Kin selection presupposes and therefore cannot explain the emergence of a coherent family unit.