Many species have a social structure in which individuals form groups and interaction among members within each group is much more frequent than interaction of individuals across groups. When selection for a biological trait, often altruism, in such populations depends on the difference between groups rather than individual differences within a group, it is described as "group selection" in evolutionary biology. Group selection in this sense is often called multi-level selection, because it posits social organization with a complexity above simple interactions of individuals. Group selection theories argue that a behavior may spread in a population because of the benefits they bestow on groups even though they cause the individuals who exhibit this behavior sacrifice fitness by aiding the 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), historically cast serious doubt on group selection as a major mechanism of evolution. However, some scientists have pursued the idea over the last few decades, and group selection models have seen a resurgence since the mid-1990s with increasing popularity.
Group selection is possible when the aggregation of individuals into groups with a particular social structure promotes the fitness of group members. For instance, wolves may associate in packs because this facilitates the capture of prey, and chimpanzees may live in groups because this provides defense against predators and promotes the defense of valuable territory.
More complex forms of group behavior involve individuals sacrificing personal fitness on behalf of other members of the group, as when a sterile soldier termite self-sacrifices to protect the nest. As long as the degree of relatedness in the nest is sufficiently great, this form of self-sacrifice can evolve, and is known as biological altruism. Some mosquito-transmitted rabbit viruses, for instance, are only transmitted to uninfected rabbits from infected rabbits that 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 nature, such viruses are much less virulent than mutants of the same viruses that in laboratory culture readily out-compete non-virulent variants (or than tick-transmitted viruses, since ticks, unlike mosquitoes, do bite dead rabbits).
Theoretical models of the 1960s suggested that group selection involving altruism is unlikely to evolve. However, these models assumed that group selection must be selection among groups, when in fact group selection is the selection at the level of the species' genome for the formation of groups. Viewed in this way, there is no conflict whatever between individual selection and group selection.
Experimental results starting in the late 1970s demonstrated that group selection involving altruism was often a powerful evolutionary force. The early group selection models were flawed, it is now known, because they assumed that genes acted independently, whereas in fact genetically based interactions among individuals are ubiquitous in group formation because genes must cooperate for the benefit of association in groups to enhance the fitness of group members (e.g.). As a result, biologists now recognize that group selection involving altruistic behavior is important in evolution (e.g., Wade et al., 2010).
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. Specifically, the parasites do not individually moderate their transmission; rather, more transmissible variants "continually arise and grow rapidly for many generations but eventually go extinct before dominating the system."
A variant of group selection theory rooted from the idea of population viscosity (limited offspring dispersal), first proposed by Hamilton (1964), that is widely present in natural populations. This population structure builds a continuum between individual selection, kin selection, kin group selection and group selection without a clear boundary for each level. However, early theoretical models by D.S. Wilson et al. (1992) and Taylor (1992) showed that pure population viscosity cannot lead to cooperation/altruism, because any benefit generated by kin cooperation is exactly cancelled out by kin competition because additional offspring resulted from cooperation cannot be exported well and will be eliminated by local competition. However, this exact cancelling out also suggests that any additional benefit of local cooperation would be sufficient for the evolution of cooperation. Mitteldorf and D.S. Wilson (2000) later showed that if the population is allowed to fluctuate, then local populations can temporarily store the benefit of local cooperation and promote the evolution of cooperation/altruism. By assuming individual differences in adaptations, Yang (2013) further showed that the benefit of local altruism can be stored in the form of offspring quality and thus promote the evolution of altruism even if the population does not fluctuate, this is because local competition among more individuals resulted from local altruism will result in an increased average local fitness of survived individuals after the elimination of less adapted individuals by natural selection.
The haystack model and trait groups
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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. One 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 is also known as 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 occasionally all the members of all the haystacks concurrently 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.
Multilevel selection theory
By 1994, the limitations of earlier models have been addressed, and newer models suggest that selection may sometimes act above the gene level. Recently David Sloan Wilson and Elliott Sober have argued that the case against group selection has been overstated. They focus their argument on whether groups can have functional organization in the same way individuals do and, consequently, whether groups can also be "vehicles" for selection. For example, groups that cooperate better may have out-reproduced those that did not. Resurrected in this way, Wilson & Sober's new group selection is usually called multilevel selection theory.
David Sloan Wilson, the developer of Multilevel Selection Theory (MLS) compares the many layers of competition and evolution to the "Russian Matryoska Dolls" within one another. The lowest level is the genes, next come the cells, and then the organism level and finally the groups. The different levels function cohesively to maximize fitness, or reproductive success. After establishing these levels, MLS goes further by saying that selection for the group level, which is competition between groups, must outweigh the individual level, which is individuals competing within a group, for a group-beneficiating trait to spread. MLS theory focuses on the phenotype this way because it looks at the levels that selection directly acts upon.
MLS theory does not lean towards individual or group selection but can be used to evaluate the balance between group selection and individual selection on a case-by-case scenario. Some experiments done imply that group selection can prevail, such as the experiment conducted by William Muir of Purdue University comparing egg productivity in hens. In the experiment, he demonstrates the existence of group selection by showing that in individual selection, a hyper-aggressive strain had been produced that led to many fatal attacks after only six generations. Group selection has been most often postulated in humans and, notably, social insects that make cooperation a driving force of their adaptations over time.
For humans, a highly pro-social, cognitive thinking species, social norms can be seen as a means of reducing the individual level variation and competition and shift selection in humans to the group level. Wilson ties the MLS theory regarding humans to another upcoming theory known as gene-culture evolution by acknowledging that culture seems to characterize a group-level mechanism for human groups to adapt to environmental changes. Methods of testing MLS include social psychological experimentation and multilevel modeling equations.
Wilson & Sober's work has been part of a revival of interest in multilevel selection as an explanation for evolutionary phenomena. Indeed, in a 2005 article, E. O. Wilson argued that kin selection could no longer be thought of as underlying the evolution of extreme sociality, for two reasons. First, some authors have shown that the argument that haplodiploid inheritance, characteristic of the Hymenoptera, creates a strong selection pressure towards nonreproductive castes is mathematically flawed. Second, eusociality no longer seems to be confined to the hymenopterans; increasing numbers of highly social taxa have been found in the years since Wilson's foundational text on sociobiology was published in 1975, including a variety of insect species, as well as a rodent species (the naked mole rat). Wilson suggests the equation for Hamilton's rule:
- rb > c
(where b represents the benefit to the recipient of altruism, c the cost to the altruist, and r their degree of relatedness) should be replaced by the more general equation
- rbk + be > c
in which bk is the benefit to kin (b in the original equation) and be is the benefit accruing to the group as a whole. He then argues that, in the present state of the evidence in relation to social insects, it appears that be>rbk, so that altruism needs to be explained in terms of selection at the colony level rather than at the kin level. However, it is well understood in social evolution theory that kin selection and group selection are not distinct processes, and that the effects of multi-level selection are already fully accounted for in Hamilton's original rule, rb>c, provided that an expanded definition of r, not requiring Hamilton's original assumption of direct genealogical relatedness, is used.
Group selection indicated by gene-culture coevolution
Gene-culture coevolution is a modern hypothesis (applicable mostly to humans) that combines evolutionary biology and modern sociobiology to indicate group selection. It treats culture as a separate evolutionary system that acts in parallel to the usual genetic evolution to transform human traits. It is believed that this approach of combining genetic influence with cultural influence over several generations is not present in the other hypotheses such as reciprocal altruism and kin selection, making gene-culture evolution one of the strongest realistic hypotheses for group selection. Fehr provides evidence of group selection taking place in humans presently with experimentation through logic games such as prisoner’s dilemma, the type of thinking that humans have developed many generations ago.
Gene-culture coevolution, or cumulative cultural evolution, allows humans to culturally evolve highly distinct adaptations to the local pressures and environments much quicker than with genetic evolution alone. Robert Boyd and Peter J. Richerson, two strong proponents of cultural evolution, postulate that the act of social learning, or learning in a group as done in group selection, allows human populations to accrue information over many generations. This leads to the cultural evolution of highly adaptive behaviors and technology alongside genetic evolution. Specifically, they believe that the ability to collaborate with each other evolved during the Middle Pleistocene, a million years ago, in response to a rapidly changing climate.
Herbert Gintis examines cultural evolution of group selection with a more statistical method, offering evidence that societies that promote pro-social norms, as in group selection, have higher survival rates than societies that do not. He does so by developing a multilevel gene-culture coevolutionary model that explains the process whereby altruistic social norms will hinder socially harmful and fitness reducing norms and consequently will be internalized. In his equations, he differentiates between a genetic group selection model that is sensitive to group size and migration rates versus his own model that is much less affected by these constraints and therefore more accurate.
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 Evolutionarily Stable Strategy (ESS), 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 (see, for instance, the ESSs created by Koinophilia).
Implications in Population Biology
While group selection by traditional definition remains a debated topic, both multilevel and kinship selection theories can play an integral role in the study of population dynamics. Methods that explore kinship selection strive to determine the optimal traits for the overall fitness of the population. Multilevel selection approaches explore processes by which selection changes the traits of a population. When both methods are investigated in conjunction, they can be used to “obtain a complete understanding of the evolution of a social behavior system”.
Social behaviors such as altruism and group relationships can impact many aspects of population dynamics, such as intraspecific competition and interspecific interactions. In 1871, Darwin argued that group selection will occur when the benefits of cooperation or altruism between subpopulations are greater than the individual benefits of egotism within a subpopulation. This supports the idea of multilevel selection, but kinship also plays an integral role because many subpopulations are composed of closely related individuals. An example of this can be found in lions, which are simultaneously cooperative and territorial. Within a pride, males protect the pride from outside males, and females, who are commonly sisters, communally raise cubs and hunt. However, this cooperation seems to be density dependent. When resources are limited, group selection will favor the prides that work together to hunt. When prey is abundant, cooperation is no longer beneficial enough to outweigh the disadvantages of altruism, and hunting is no longer a cooperative effort.
Interactions between different species can also be affected by multilevel selection. The previously mentioned example of how group selection works on mosquito-borne viruses in rabbits demonstrates how parasite-host interactions are impacted. Predator-prey relationships can also be affected. Individuals of certain monkey species will howl to warn the group of the approach of a predator. The evolution of this trait benefits the group by providing protection, but acts a major disadvantageto the individual because the howling draws the predator’s attention to them. By affecting these interspecific interactions, multilevel and kinship selectioncan change the overall population dynamics of an ecosystem.
Multilevel selection is often applied to explain the evolution of altruistic behaviors in terms of quantitative genetics. Increased frequency or fixation of altruistic alleles can be accomplished through kinship selection, in which individuals engage in altruistic behaviors to promote the fitness of genetically similar individuals such as siblings. However, this can lead to inbreeding and inbreeding depression, which typically lowers the overall fitness of a population.However, if altruism were to be selected for through an emphasis on benefit to the group as opposed to relatedness and benefit to kin, both the altruistic trait and genetic diversity could be preserved. However, relatedness should still remain a key consideration in studies of multilevel selection. Experimentally imposed multilevel selection on Japanese quail was more effective by an order of magnitude when performed on closely related kin groups than when performed on randomized groups of individuals. This further demonstrates the importance of an understanding of the relationships and interactions of all levels of group selection.
The use of the Price equation to support group selection has been recently challenged by van Veelen et al. (2012). They suggested that the Price equation is based on certain invalid mathematical assumptions.
Group selection isn’t widely accepted by evolutionists for several reasons. First, it’s not an efficient way to select for traits, like altruistic behavior, that are supposed to be detrimental to the individual but good for the group. Groups divide to form other groups much less often than organisms reproduce to form other organisms, so group selection for altruism would be unlikely to override the tendency of each group to quickly lose its altruists through natural selection favoring cheaters. Further, little evidence exists that selection on groups has promoted the evolution of any trait. Finally, other, more plausible evolutionary forces, like direct selection on individuals for reciprocal support, could have made humans prosocial. These reasons explain why only a few biologists, like [David Sloan] Wilson and E. O. Wilson (no relation), advocate group selection as the evolutionary source of cooperation.
Richard Dawkins and fellow advocates of the gene-centered view of evolution remain unconvinced about group selection. In particular, Dawkins suggests that group selection fails to make an appropriate distinction between replicators and vehicles. Psychologist Steven Pinker concluded that "Group Selection has no useful role to play in psychology or social science." 
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