Group Selection - The Haystack Model and Trait Groups

The Haystack Model and Trait Groups

Maynard Smith can be credited with what has become known as the "haystack model" of group selection. As a non-mathematical introduction to the idea, imagine a group of animals that spend most of their time living and breeding in haystacks but that occasionally all come out of their haystacks simultaneously, mix together and then separate into equal groups, which once again go off to inhabit separate haystacks. We can then imagine a trait that benefits each haystack group, perhaps leading to behaviorally altruistic acts that cost an individual some fitness but enhance the fitness of its group even more, and a selfish trait that, for the purposes of this discussion, we can call the absence of the altruistic trait.

Each of these two traits works on a different level of selection. Within the individual haystacks the selfish organisms benefit in terms of evolutionary fitness. This is because the selfish organisms benefit from the actions of the altruistic organisms but do not pay any of the evolutionary costs for being altruistic (sacrificing some good for that of others). Thus, in each generation the number of altruists in the group would shrink compared to the number of selfish organisms. As a result one might first think that a group beneficial trait, especially an altruistic one, would be doomed to eventually die out. But we must remember the strange nature of these hypothetical organisms. Every so often, at the same time, all the members of all the haystacks form one large group, randomly assort into equal groups, and then move back into the haystacks. Because of this an altruistic behavior can take hold by the following reasoning. While the number of selfish organisms in each haystack increases in percentage every generation, the total population of haystacks that contain altruists produce more offspring over all than those that do not. This means that populations with altruists are going to be over-represented when all the haystacks are abandoned to form a larger group. So long as the number of generations spent in each haystack is not so long as to dramatically reduce the number of altruists, and so long as the group benefit of the altruistic trait is significant enough, the number of altruists in all the haystack populations can rise.

However, though Maynard Smith gave a mathematical model by which group selection might work, he was skeptical that it would happen in nature often enough to be worth considering. His reasoning was that the specific conditions for group selection to take hold, namely the repeated isolation, mixture, and reisolation of organisms would be so rare and unlikely to occur in nature that it was almost certainly not a significant evolutionary force.

In their 1998 book Unto Others, and in various articles before this, Elliott Sober and David Sloan Wilson challenge this view. While one of their challenges takes the form of naming organisms, such as the so called "brain worm" (Dicrocoelium dendriticum), which has a life cycle much like that of the haystack organisms above, they present a more significant argument, based on the notion of trait groups.

Trait groups can occur within larger groups through the interaction of particular genetic traits, and need not interact for a generation to promote survival value. Sober and Wilson see kin selection, which is often considered an alternative to group selection, as a special case of a trait groups. To see how a trait group could be beneficial, let's imagine an altruist trait, such as cooperation with another organism even in such cases where it only benefits 40% as much as the organism it helps, and a selfish trait such as cooperating with another organism only when it will benefit more than the organism it helps. The first trait is considered altruistic in Sober and Wilson’s sense because the within-group fitness of the altruistic organism drops every time it cooperates compared with the other member of the group. Now imagine five organisms, one of which is altruistic in regards to this trait, and the rest of which are selfish. Assume that each case of cooperation increases the chance of survival and reproduction by 10 units, which is divided among the interacting pair (group of two). Now assume that member of the population groups/interacts with each other member of the population one time. After all the interactions have taken place, the selfish organisms have each acquired 6 units. This is because they all refuse to cooperation with other selfish members (since it is impossible for both members to benefit more than the other), but each takes advantage of the altruist benefits over that individual in a ratio of 60% to 40%. The altruist on the other hand has interacted with 4 selfish organisms and thus has earned 16 units (four for each encounter) and thus has a greater survival advantage than the selfish members of the population. The altruist ends up winning the survival "war" even though it came out behind in every survival "battle".

Because individuals can form hundreds or even thousands of trait groups within its life span, the trait group selection model does not have to rely on the unlikely situation of an entire population isolating into groups, merging, and then isolating into groups again. Likewise the rate at which trait groups can form and dissolve can be many times faster than the rate at which individuals reproduce, providing cumulative as opposed to all-or-nothing benefits. It is important to note that this argument has not settled the issue of group selection however. There is still heavy debate as to whether or not such formations count as "real" groups in the traditional biological sense of groups affected by group selection.