Group selection
Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene.
Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behaviour of animals could affect their survival and reproduction as groups, speaking for instance of actions for the good of the species. In the 1930s, Ronald Fisher and J. B. S. Haldane proposed the concept of kin selection, a form of biological altruism from the gene-centered view of evolution, arguing that animals should sacrifice for their relatives, and thereby implying that they should not sacrifice for non-relatives. From the mid-1960s, evolutionary biologists such as John Maynard Smith, W. D. Hamilton, George C. Williams, and Richard Dawkins argued that natural selection acts primarily at the level of the gene. They argued on the basis of mathematical models that individuals would not altruistically sacrifice fitness for the sake of a group unless it would ultimately increase the likelihood of an individual passing on their genes. A consensus emerged that group selection did not occur, including in special situations such as the haplodiploid social insects like honeybees, where kin selection explains the behaviour of non-reproductives equally well, since the only way for them to reproduce their genes is via kin.[2]
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.[3] They argued that group selection can occur when competition between two or more groups, some containing altruistic individuals who act cooperatively together, is more important for survival than competition between individuals within each group.[3] A large group of ethologists conceded that while inclusive fitness may be debatable, it was still a useful theory in practice.[2] However, the vast majority of behavioural biologists have not been convinced by renewed attempts to revisit group selection as a plausible mechanism of evolution.[4]
Background
[edit]Charles Darwin developed the theory of evolution in his book, Origin of Species. Darwin also made the first suggestion of group selection in The Descent of Man that the evolution of groups could affect the survival of individuals. He wrote, "If one man in a tribe... invented a new snare or weapon, the tribe would increase in number, spread, and supplant other tribes. In a tribe thus rendered more numerous there would always be a rather better chance of the birth of other superior and inventive members."[5][6]
Once Darwinism had been accepted in the modern synthesis of the mid-twentieth century, animal behaviour 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 behaviour patterns were "for the good of the species",[1][7] without actually studying survival value in the field.[7] The evolutionary biologist Richard Dawkins wrote that Lorenz was a "'good of the species' man",[8] so accustomed to group selection thinking that he did not realize his views "contravened orthodox Darwinian theory".[8] The ethologist Niko Tinbergen praised Lorenz for his interest in the survival value of behaviour, and naturalists enjoyed Lorenz's writings for the same reason.[7] In 1962, group selection was used as a popular explanation for adaptation by the zoologist V. C. Wynne-Edwards.[9][10] In 1976, Dawkins wrote a well-known book on the importance of evolution at the level of the gene or the individual, The Selfish Gene.[11]
From the mid-1960s, evolutionary biologists argued that natural selection acted primarily at the level of the individual. In 1964, John Maynard Smith,[12] C. M. Perrins (1964),[13] 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.[14][15]
It was in the 1960s generally agreed that group selection also applied for eusocial insects such as honeybees.[2]
Kin selection and inclusive fitness theory
[edit]Theory makes group selection difficult
[edit]
Experiments from the late 1970s suggested that selection involving groups was possible.[16] Early group selection models assumed that genes acted independently, for example a gene that coded for cooperation or altruism. Genetically based reproduction of individuals implies that, in group formation, the altruistic genes would need a way to act for the benefit of members in the group to enhance the fitness of many individuals with the same gene.[17] But it is expected from this model that individuals of the same species would compete against each other for the same resources. This would put cooperating individuals at a disadvantage, making genes for cooperation likely to be eliminated.[11][18] Group selection on the level of the species is flawed because it is difficult to see how selective pressures would be applied to competing/non-cooperating individuals.[11]
Alternative explanation of altruism
[edit]Unlike group selection, kin selection between related individuals is accepted by most biologists as an explanation of altruistic behaviour.[2] R.A. Fisher in 1930[19] and J.B.S. Haldane in 1932[20] set out the mathematics of kin selection, with Haldane famously joking that he would willingly die for two brothers or eight cousins.[21] In this model, genetically related individuals cooperate because survival advantages to one individual also benefit kin who share some fraction of the same genes, giving a mechanism for selection in favour of this much altruism, without involving group selection.[22]
Inclusive fitness theory, first proposed by W. D. Hamilton in the early 1960s, gives a selection criterion for evolution of social traits when social behaviour is costly to an individual organism's survival and reproduction. The criterion is that the reproductive benefit to relatives who carry the social trait, multiplied by their relatedness (the probability that they share the altruistic trait) exceeds the cost to the individual. Inclusive fitness theory is a general treatment of the statistical probabilities of social traits accruing to any other organisms likely to propagate a copy of the same social trait. Kin selection theory treats the narrower but simpler case of the benefits to close genetic relatives (or what biologists call 'kin') who may also carry and propagate the trait. The theory is widely accepted by biologists.[2]
Kin recognition
[edit]One of the questions about kin selection is the requirement that individuals must know if other individuals are related to them, or kin recognition. Any altruistic act has to preserve similar genes. One argument given by Hamilton is that many individuals operate in "viscous" conditions, so that they live in physical proximity to relatives. Under these conditions, they can act altruistically to any other individual, and it is likely that the other individual will be related. This population structure builds a continuum between individual selection, kin selection, kin group selection and group selection without clear boundaries between these. However, early theoretical models by D. S. Wilson et al.[23] and P. D. Taylor[24] 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. Mitteldorf and D. S. Wilson later showed that if the population is allowed to fluctuate, local populations can temporarily store the benefit of local cooperation and promote the evolution of cooperation and altruism.[25] By assuming individual differences in adaptations, Jiang-Nan Yang 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.[26]
Reciprocal altruism
[edit]Another explanation for the recognition of genes for altruism is that a single trait, reciprocal altruism, is capable of explaining the vast majority of altruism that is generally accepted as "good" by modern societies. The phenotype of altruism relies on recognition of the altruistic behaviour by itself. The trait of kindness is recognized by sufficiently intelligent and undeceived organisms in other individuals with the same trait. Moreover, the existence of such a trait predicts a tendency for kindness to unrelated organisms that are apparently kind, even if the organisms are of another species. The gene need not be exactly the same, so long as the effect or phenotype is similar. Multiple versions of the gene—or even meme—would have virtually the same effect. This explanation was given by Richard Dawkins as an analogy of a man with a green beard. Green-bearded men are imagined as tending to cooperate with each other simply by seeing a green beard, where the green beard trait is incidentally linked to the reciprocal kindness trait.[11]
Multilevel selection theory
[edit]Context
[edit]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.[17] Additionally, group selection at the species level 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, would be more plausible. In contrast, kin selection is accepted as an explanation of altruistic behaviour.[22][27] The biologist Charles Goodnight argues that kin selection and multilevel selection are both needed to "obtain a complete understanding of the evolution of a social behavior system".[28]
A revived group selection theory
[edit]In 1994, the evolutionary biologist David Sloan Wilson and the philosopher of biology 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, D. S. Wilson & Sober's new group selection is called multilevel selection theory.[29]
D. S. Wilson compared the layers of competition and evolution to nested sets of Russian matryoshka dolls.[30] 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-benefiting trait to spread.[31]
Multilevel selection theory focuses on the phenotype because it looks at the levels that selection directly acts upon.[30] For humans, social norms can be argued to reduce individual level variation and competition, thus shifting selection to the group level. The assumption is that variation between different groups is larger than variation within groups. Competition and selection can operate at all levels regardless of scale. D. S. Wilson wrote, "At all scales, there must be mechanisms that coordinate the right kinds of action and prevent disruptive forms of self-serving behavior at lower levels of social organization."[32] E. O. Wilson summarized, "In a group, selfish individuals beat altruistic individuals. But, groups of altruistic individuals beat groups of selfish individuals."[33]
D. S. Wilson argues that while kin selection works well for the behaviour of many animals, human behaviour is difficult to explain using kin selection alone. In particular, he claims it does not explain the rapid rise of human civilization, and that other factors must be considered.[32] He and others have continued to develop group selection models.[26][34][28] He 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.[31]
D. S. Wilson and Sober's work revived interest in multilevel selection. 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, he suggested, the argument that haplodiploid inheritance (as in the Hymenoptera) creates a strong selection pressure towards nonreproductive castes is mathematically flawed.[35][36] Second, eusociality no longer seems to be confined to the hymenopterans; increasing numbers of highly social taxa have been found in the years since E. O. Wilson's foundational text Sociobiology: A New Synthesis was published in 1975.[37] These including a variety of insect species, as well as two rodent species (the naked mole-rat and the Damaraland mole rat). E. O. Wilson suggests that the equation for Hamilton's rule:[38]
- 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,[39] 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.[40]
Debate in Nature
[edit]In 2010, the mathematical biologists Martin Nowak and Corina Tarnita, with the entomologist E. O. Wilson, argued in Nature for multi-level selection, including group selection, to correct what they saw as deficits in the explanatory power of inclusive fitness.[3] 137 other evolutionary biologists, also in Nature, responded "that their arguments are based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature".[41]
Proposed applications
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Multi-level selection theory is proposed as suitable for evaluating the balance between group selection and individual selection in specific cases.[31] 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.[42] Group selection has most often been postulated in humans[43] and in eusocial