Group_selection

Group selection

In evolutionary biology, group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group.

Group selection was used as a popular explanation for adaptations, especially by V. C. Wynne-Edwards. For several decades, however, critiques, particularly by George C. Williams, John Maynard Smith and C.M. Perrins (1964), cast serious doubt on group selection as a major mechanism of evolution, and though some scientists have pursued the idea over the last few decades, only recently have group selection models seen a minor resurgence (albeit not as a fundamental mechanism but as a phenomenon emergent from standard selection ).

Overview

Specific syndromes of selective factors can create situations in which groups are selected because they display group properties which are selected-for. Some mosquito-transmitted rabbit viruses, for instance, are only transmitted to uninfected rabbits from infected rabbits which are still alive. This creates a selective pressure on every group of viruses already infecting a rabbit not to become too virulent and kill their host rabbit before enough mosquitoes have bitten it. In natural systems such viruses display much lower virulence levels than do mutants of the same viruses that in laboratory culture readily out-compete non-virulent variants (or than do tick-transmitted viruses—ticks, unlike mosquitoes, bite dead rabbits).

However, theoretical models of the 1960s seemed to imply that the effect of group selection was negligible. Alleles are likely to be held on a population-wide level, leaving nothing for group selection to select for. Additionally, generation time is much longer for groups than it is for individuals. Assuming conflicting selection pressures, individual selection will occur much faster, swamping any changes potentially favored by group selection. The Price equation can partition variance caused by natural selection at the individual level and the group level, and individual level selection generally causes greater effects.

Experimental results starting in the late 1970s demonstrated that group selection was far more effective than the then-current theoretical models had predicted. A review of this experimental work has shown that the early group selection models were flawed because they assumed that genes acted independently, whereas in the experimental work it was apparent that gene interaction, and more importantly, genetically based interactions among individuals, were an important source of the response to group selection (e.g. ). As a result many are beginning to recognize that group selection, or more appropriately multilevel selection, is potentially an important force in evolution.

More recently, Yaneer Bar-Yam has claimed that the gene-centered view (and thus Ronald Fisher's treatment of evolution) relies upon a mathematical approximation that is not generally valid. Bar-Yam argues that the approximation is a dynamic form of the Mean Field approximation frequently used in physics and whose limitations are recognized there. In biology, the approximation breaks down when there are spatial populations resulting in inhomogeneous genetic types (called symmetry breaking in physics). Such symmetry breaking may also correspond to speciation.

Spatial populations of predators and prey have also been shown to show restraint of reproduction at equilibrium, both individually and through social communication, as originally proposed by Wynne-Edwards. While these spatial populations do not have well-defined groups for group selection, the local spatial interactions of organisms in transient groups are sufficient to lead to a kind of multi-level selection. There is however as yet no evidence that these processes operate in the situations where Wynne-Edwards posited them; Rauch et al's analysis, for example, is of a host-parasite situation, which was recognised as one where group selection was possible even by E. O. Wilson (1975), in a treatise broadly hostile to the whole idea of group selection.

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 which spend most of their time lives living and breeding in haystacks but which occasionally all come out of their haystacks simultaneously and, mix together, and then separate into equal groups which once again go off to inhabit separate haystacks. We can then imagine trait that benefits each haystack group, for example an behaviorally altruistic act which costs the individual some fitness but enhances the fitness of its group even more, and a selfish trait which, for the purposes of this discussion, we can just 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 which contain altruists produce more offspring over all than those which do not. This means that populations with altruists are going to be over-represented when all the haystacks are abandoned for 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.

But though 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 1999 books Unto Others, and in various articles before this, Sober and 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 very much like that of the haystack organisms mentioned 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, lets imagine an altruist trait, such as cooperation with another organism even in such cases were 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.

Multilevel selection theory

Group Selection due to differing ESSs

The problem with group selection is that for a whole group to get a single trait, it must spread through the whole group first by regular evolution. But, as J. L. Mackie suggested, when there are many different groups, each with a different ESS (Evolutionary Stable Strategy), there is selection between the different ESSs, since some are worse than others. For example, a group where altruism arose would outcompete a group where every creature acted in its own interest.