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Group selection

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Early explanations of social behavior, such as the lekking of blackcock, spoke of "the good of the species".[1] Blackcocks at the Lek watercolour and bodycolour by Archibald Thorburn, 1901.

Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the more conventional level of the individual.

Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behavior of animals could affect their survival and reproduction as groups, speaking for instance of actions for the good of the species.

From the mid 1960s, evolutionary biologists such as John Maynard Smith argued that natural selection acted primarily at the level of the individual. They argued on the basis of mathematical models that individuals would not altruistically sacrifice fitness for the sake of a group. They persuaded the majority of biologists that group selection did not occur, other than in special situations such as the haplodiploid social insects like honeybees (in the Hymenoptera), where kin selection was possible.

In 1994 David Sloan Wilson and Elliott Sober argued for multi-level selection, including group selection, on the grounds that groups, like individuals, could compete. In 2010 three authors including E. O. Wilson, known for his work on social insects especially ants, again revisited the arguments for group selection, provoking a strong rebuttal from a large group of evolutionary biologists. As of yet, there is no clear consensus among biologists regarding the importance of group selection.

Good of the species

Once Darwinism had been accepted, animal behavior was glibly explained with unsubstantiated hypotheses about survival value, which was largely taken for granted. The naturalist Konrad Lorenz had argued loosely in books like On Aggression (1966) that animal behavior patterns were "for the good of the species",[1][2] without actually studying survival value in the field; the ethologist Niko Tinbergen praised Lorenz for his interest in the survival value of behavior, and naturalists enjoyed Lorenz's writings for the same reason.[2] In 1962, group selection was used as a popular explanation for adaptation by the zoologist V. C. Wynne-Edwards.[3][4]

Selfish selection

Social behavior in honeybees is explained by kin selection: their haplodiploid inheritance system makes workers very closely related to their queen (centre).
Further information: natural selection and kin selection

From the mid 1960s, evolutionary biologists argued that natural selection acted primarily at the level of the individual. In 1964, John Maynard Smith,[5] C.M. Perrins (1964),[6] and George C. Williams in his 1966 book Adaptation and Natural Selection cast serious doubt on group selection as a major mechanism of evolution; Williams's 1971 book Group Selection assembled writings from many authors on the same theme.[7][8]

The great majority of evolutionary biologists believe (2011) that selection above the level of the individual is a special case, probably limited to the unique inheritance system (involving haplodiploidy) of the eusocial Hymenoptera such as honeybees, which encourages kin selection.[9]

Group selection revisited

Further information: Unit of selection

Experiments from the late 1970s suggested that group selection involving altruism was possible.[10] Early group selection models were flawed because they assumed that genes acted independently; but 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.[11] Additionally, group selection on the level of the species is flawed because it is difficult to see how selective pressures would be applied; selection in social species of groups against other groups, rather than the species entire, seems to be the level at which selective pressures are plausible. On the other hand, kin selection is accepted as an explanation of altruistic behavior.[12]

Since the 1990s, group selection models have seen a resurgence.[13][14][15][16][17] Some biologists argue that kin selection and multilevel selection are both needed to "obtain a complete understanding of the evolution of a social behavior system".[18]

In 2010 three authors including E. O. Wilson argued for multi-level selection, including group selection, to correct what they saw as deficits in the explanatory power of inclusive fitness.[19] The response was a back-lash from 137 other evolutionary biologists who argued "that their arguments are based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature".[9]

Multilevel selection theory

In 1994 David Sloan Wilson and Elliott Sober argued that the case against group selection had been overstated. They considered whether groups can have functional organization in the same way as individuals, and consequently whether groups can be "vehicles" for selection. They do not posit evolution on the level of the species, but selective pressures that winnow out small groups within a species, e.g. groups of social insects or primates. Groups that cooperate better might survive and reproduce more than those that did not. Resurrected in this way, Wilson & Sober's new group selection is called multilevel selection theory.[20]

David Sloan Wilson and Elliott Sober's 1994 Multilevel Selection Model, illustrated by a nested set of Russian matryoshka dolls. Wilson himself compared his model to such a set.

Wilson compared the layers of competition and evolution to nested sets of Russian matryoshka dolls.[15] The lowest level is the genes, next come the cells, then the organism level and finally the groups. The different levels function cohesively to maximize fitness, or reproductive success. The theory asserts that selection for the group level, involving competition between groups, must outweigh the individual level, involving individuals competing within a group, for a group-beneficiating trait to spread.[21] Multilevel selection theory focuses on the phenotype because it looks at the levels that selection directly acts upon.[15] For humans, social norms can be argued to reduce individual level variation and competition, thus shifting selection to the group level. Wilson ties the multilevel selection theory regarding humans to another theory, gene-culture coevolution, by acknowledging that culture seems to characterize a group-level mechanism for human groups to adapt to environmental changes.[21]

MLS theory can be used to evaluate the balance between group selection and individual selection in specific cases.[21] An experiment by William Muir compared egg productivity in hens, showing that a hyper-aggressive strain had been produced through individual selection, leading to many fatal attacks after only six generations; by implication, it could be argued that group selection must have been acting to prevent this in real life.[22] Group selection has most often been postulated in humans and, notably, eusocial Hymenoptera that make cooperation a driving force of their adaptations over time and have a unique system of inheritance involving haplodiploidy that allows the colony to function as an individual while only the queen reproduces.[23]

Wilson & Sober's work revived interest in multilevel selection. In a 2005 article,[24] E. O. Wilson argued that kin selection could no longer be thought of as underlying the evolution of extreme sociality, for two reasons. First, he suggested, the argument that haplodiploid inheritance (as in the Hymenoptera) creates a strong selection pressure towards nonreproductive castes is mathematically flawed.[25] 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,[26] including a variety of insect species, as well as two rodent species (the naked mole-rat and the Damaraland mole rat). Wilson suggests the equation for Hamilton's rule:[27]

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, kin selection and group selection are not distinct processes, and the effects of multi-level selection are already accounted for in Hamilton's rule, rb>c,[28] provided that an expanded definition of r, not requiring Hamilton's original assumption of direct genealogical relatedness, is used, as proposed by E. O. Wilson himself.[29]

Other arguments

Spatial populations of predators and prey show restraint of reproduction at equilibrium, both individually[30] and through social communication,[31] 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,[30] 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.[26] 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 in the idea of population viscosity (limited offspring dispersal), first proposed by Hamilton (1964), 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)[32] and Taylor (1992)[33] showed that pure population viscosity cannot lead to cooperation and altruism. This is because any benefit generated by kin cooperation is exactly cancelled out by kin competition; additional offspring from cooperation are eliminated by local competition. However, this exact cancelling out also suggests that any additional benefit of local cooperation would be sufficient to allow cooperation to evolve.[16] 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 and altruism.[34] 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 resulting from local altruism increases the average local fitness of the individuals that survive.[16]

Gene-culture coevolution in humans

Main articles: Dual inheritance theory and cultural group selection
Humanity has developed extremely rapidly, arguably through gene-culture coevolution, leading to complex cultural artefacts like the gopuram of the Sri Mariammam temple, Singapore.

Gene-culture coevolution (also called dual inheritance theory) is a modern hypothesis (applicable mostly to humans) that combines evolutionary biology and modern sociobiology to indicate group selection.[35] 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.[36]

Gene-culture coevolution allows humans to develop highly distinct adaptations to the local pressures and environments more quickly 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.[37] This leads to cultural evolution of behaviors and technology alongside genetic evolution. Boyd and Richerson believe that the ability to collaborate evolved during the Middle Pleistocene, a million years ago, in response to a rapidly changing climate.[37]

In 2003, Herbert Gintis examined cultural evolution statistically, offering evidence that societies that promote pro-social norms have higher survival rates than societies that do not.[38][39]

Differing evolutionarily stable strategies (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.[40] For example, a group where altruism was universal would outcompete a group where every creature acted in its own interest, but a mixed group of altruists and non-altruists would be vulnerable to cheating by non-altruists within the group.[41]

Implications in population biology

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 occurs when the benefits of cooperation or altruism between subpopulations are greater than the individual benefits of egotism within a subpopulation.[42] 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.[43] 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 favors 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 cooperative.[43]

Interactions between different species can also be affected by multilevel selection. Predator-prey relationships can also be affected. Individuals of certain monkey species howl to warn the group of the approach of a predator.[44] The evolution of this trait benefits the group by providing protection, but could be disadvantageous to the individual if the howling draws the predator's attention to them. By affecting these interspecific interactions, multilevel and kinship selection can change the population dynamics of an ecosystem.[44]

Multilevel selection attempts to explain the evolution of altruistic behavior in terms of quantitative genetics. Increased frequency or fixation of altruistic alleles can be accomplished through kin selection, in which individuals engage in altruistic behavior to promote the fitness of genetically similar individuals such as siblings. However, this can lead to inbreeding depression,[45] 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 on closely related kin groups than on randomized groups of individuals.[46]

Criticism

The use of the Price equation to support group selection was challenged by van Veelen in 2012, arguing that it is based on invalid mathematical assumptions.[47]

Richard Dawkins and other advocates of the gene-centered view of evolution remain unconvinced about group selection.[48][49][50] In particular, Dawkins suggests that group selection fails to make an appropriate distinction between replicators and vehicles.[51]

The psychologist Steven Pinker concluded that "group selection has no useful role to play in psychology or social science", as it "is not a precise implementation of the theory of natural selection, as it is, say, in genetic algorithms or artificial life simulations. Instead it is a loose metaphor, more like the struggle among kinds of tires or telephones."[52]

The evolutionary biologist Jerry Coyne summarized the arguments in The New York Times in non-technical terms as follows:[53]

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.[53]

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Further reading

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