Eusociality
Eusociality (
Eusociality has evolved among the
History
The term "eusocial" was introduced in 1966 by
In 1969,
- Egg-layers and worker-like individuals among adult females (division of labor)
- The overlap of generations (mother and adult offspring)
- Cooperative work on the cells of the bees' honeycomb
E. O. Wilson extended the terminology to include other social insects, such as ants, wasps, and termites. Originally, it was defined to include organisms (only invertebrates) that had the following three features:[1][5][6][7]
- Reproductive division of labor (with or without sterile castes)
- Overlapping generations
- Cooperative care of young
Eusociality was then discovered in a group of
Diversity
Most eusocial societies exist in arthropods, while a few are found in mammals. Some ferns may exhibit a primitive form of eusocial behavior.[8][9]
In insects
Eusociality has evolved multiple times in different insect orders, including hymenopterans,[10] termites,[11] thrips,[12] aphids,[12] and beetles.[13]
In hymenoptera
The order Hymenoptera contains the largest group of eusocial insects, including ants, bees, and wasps—divided into castes: reproductive queens, drones, more or less sterile workers, and sometimes also soldiers that perform specialized tasks.[14] In the well-studied social wasp Polistes versicolor,[15] dominant females perform tasks such as building new cells and ovipositing, while subordinate females tend to perform tasks like feeding the larvae and foraging. The task differentiation between castes can be seen in the fact that subordinates complete 81.4% of the total foraging activity, while dominants only complete 18.6% of the total foraging.[16] Eusocial species with a sterile caste are sometimes called hypersocial.[17]
While only a moderate percentage of species in bees (families
Reproductive specialization generally involves the production of sterile members of the species, which carry out specialized tasks to care for the reproductive members. Individuals may have behavior and morphology modified for group defense, including
In Lasioglossum aeneiventre, a halictid bee from Central America, nests may be headed by more than one female; such nests have more cells, and the number of active cells per female is correlated with the number of females in the nest, implying that having more females leads to more efficient building and provisioning of cells.[25] In similar species with only one queen, such as Lasioglossum malachurum in Europe, the degree of eusociality depends on the clime in which the species is found.[26]
In termites
In beetles
In gall-inducing insects
Some
In crustaceans
Eusociality has evolved in three different lineages in the colonial
The fortress defense hypothesis additionally points out that because sponges provide both food and shelter, there is an aggregation of relatives (because the shrimp do not have to disperse to find food), and much competition for those nesting sites. Being the target of attack promotes a good defense system (soldier caste); soldiers promote the fitness of the whole nest by ensuring safety and reproduction of the queen.[35]
Eusociality offers a competitive advantage in shrimp populations. Eusocial species are more abundant, occupy more of the habitat, and use more of the available resources than non-eusocial species.[36][37][38]
In nonhuman mammals
Among mammals, two species in the rodent group
Some mammals in the
In humans
Scientists have debated whether humans are
Though controversial,
In plants
One plant, the epiphytic staghorn fern, Platycerium bifurcatum (Polypodiaceae), may exhibit a primitive form of eusocial behavior amongst clones. The evidence for this is that individuals live in colonies, where they are structured in different ways, with fronds of differing size and shape, to collect and store water and nutrients for the colony to use. At the top of a colony, there are both pleated fan-shaped "nest" fronds that collect and hold water, and gutter-shaped "strap" fronds that channel water: no solitary Platycerium species has both types. At the bottom of a colony, there are "nest" fronds that clasp the trunk of the tree supporting the fern, and drooping photosynthetic fronds. These are argued to be adapted to support the colony structurally, i.e. that the individuals in the colony are to some degree specialized for tasks, a division of labor.[8][9][59]
Evolution
Phylogenetic distribution
Eusociality is a rare but widespread phenomenon in species in at least seven orders in the animal kingdom, as shown in the phylogenetic tree (non-eusocial groups not shown). All species of termites are eusocial, and it is believed that they were the first eusocial animals to evolve, sometime in the upper Jurassic period (~150 million years ago).[60] The other orders shown contain both eusocial and non-eusocial species, including many lineages where eusociality is inferred to be the ancestral state. Thus the number of independent evolutions of eusociality (clades) is not known. The major eusocial groups are shown in boldface in the phylogenetic tree.
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Paradox
Prior to the
Inclusive fitness and haplodiploidy
Argument that haplodiploidy favors eusociality
According to
In haplodiploid species, females develop from fertilized eggs and males develop from unfertilized eggs. Because a male is haploid, his daughters share 100% of his genes and 50% of their mother's. Therefore, they share 75% of their genes with each other. This mechanism of sex determination gives rise to what W. D. Hamilton first termed "supersisters", more closely related to their sisters than they would be to their own offspring.[65] Even though workers often do not reproduce, they can pass on more of their genes by helping to raise their sisters than by having their own offspring (each of which would only have 50% of their genes). This unusual situation, where females may have greater fitness when they help rear sisters rather than producing offspring, is often invoked to explain the multiple independent evolutions of eusociality (at least nine separate times) within the Hymenoptera.[66]
Argument that haplodiploidy does not favor eusociality
Against the supposed benefits of haplodiploidy for eusociality, Robert Trivers notes that while females share 75% of genes with their sisters in haplodiploid populations, they only share 25% of their genes with their brothers.[67] Accordingly, the average relatedness of an individual to their sibling is 50%. Therefore, helping behavior is only advantageous if it is biased to helping sisters, which would drive the population to a 1:3 sex ratio of males to females. At this ratio, males, as the rarer sex, increase in reproductive value, negating the benefit of female-biased investment.[68]
Further, not all eusocial species are haplodiploid: termites, some snapping shrimps, and mole rats are not. Conversely, many non-eusocial bees are haplodiploid, and among eusocial species many queens mate with multiple males, resulting in a hive of half-sisters that share only 25% of their genes. The association between haplodiploidy and eusociality is below statistical significance.[69] Haplodiploidy is thus neither necessary nor sufficient for eusociality to emerge.[70] Relatedness does still play a part, as monogamy (queens mating singly) is the ancestral state for all eusocial species so far investigated.[71] If kin selection is an important force driving the evolution of eusociality, monogamy should be the ancestral state, because it maximizes the relatedness of colony members.[71]
Evolutionary ecology
Increased parasitism and predation rates are the primary ecological drivers of social organization. Group living affords colony members defense against enemies, specifically predators, parasites, and competitors, and allows them to gain advantage from superior foraging methods.[7] The importance of ecology in the evolution of eusociality is supported by evidence such as experimentally induced reproductive division of labor, for example when normally solitary queens are forced together.[72] Conversely, female Damaraland mole-rats undergo hormonal changes that promote dispersal after periods of high rainfall.[73]
Climate too appears to be a selective agent driving social complexity; across bee lineages and Hymenoptera in general, higher forms of sociality are more likely to occur in tropical than temperate environments.[74] Similarly, social transitions within halictid bees, where eusociality has been gained and lost multiple times, are correlated with periods of climatic warming. Social behavior in facultative social bees is often reliably predicted by ecological conditions, and switches in behavioral type have been experimentally induced by translocating offspring of solitary or social populations to warm and cool climates. In H. rubicundus, females produce a single brood in cooler regions and two or more broods in warmer regions, so the former populations are solitary while the latter are social.[75] In another species of sweat bees, L. calceatum, social phenotype has been predicted by altitude and micro-habitat composition, with social nests found in warmer, sunnier sites, and solitary nests found in adjacent, cooler, shaded locations. Facultatively social bee species, however, which comprise the majority of social bee diversity, have their lowest diversity in the tropics, being largely limited to temperate regions.[76]
Multilevel selection
Once pre-adaptations such as group formation, nest building, high cost of dispersal, and morphological variation are present, between-group competition has been suggested as a driver of the transition to advanced eusociality. M. A. Nowak, C. E. Tarnita, and E. O. Wilson proposed in 2010 that since eusociality produces an extremely altruistic society, eusocial groups should out-reproduce their less cooperative competitors, eventually eliminating all non-eusocial groups from a species.[77] Multilevel selection has been heavily criticized for its conflict with the kin selection theory.[78]
Reversal to solitarity
A reversal to solitarity is an evolutionary phenomenon in which descendants of a eusocial group evolve solitary behavior once again. Bees have been model organisms for the study of reversal to solitarity, because of the diversity of their social systems. Each of the four origins of eusociality in bees was followed by at least one reversal to solitarity, giving a total of at least nine reversals.[4][5] In a few species, solitary and eusocial colonies appear simultaneously in the same population, and different populations of the same species may be fully solitary or eusocial.[75] This suggests that eusociality is costly to maintain, and can only persist when ecological variables favor it. Disadvantages of eusociality include the cost of investing in non-reproductive offspring, and an increased risk of disease.[79]
All reversals to solitarity have occurred among primitively eusocial groups; none have followed the emergence of advanced eusociality. The "point of no return" hypothesis posits that the morphological differentiation of reproductive and non-reproductive castes prevents highly eusocial species such as the honeybee from reverting to the solitary state.[20]
Physiology and development
Pheromones
The levels of two of the aliphatic compounds increase rapidly in virgin queens within the first week after
Among ants, the queen pheromone system of the fire ant
Similar mechanisms exist in the eusocial wasp Vespula vulgaris. For a queen to dominate all the workers, usually numbering more than 3000 in a colony, she signals her dominance with pheromones. The workers regularly lick the queen while feeding her, and the air-borne pheromone from the queen's body alerts those workers of her dominance.[83]
The mode of action of inhibitory pheromones which prevent the development of eggs in workers has been demonstrated in the bumble bee Bombus terrestris.[82] The pheromones suppress activity of the endocrine gland, the corpus allatum, stopping it from secreting juvenile hormone.[84] With low juvenile hormone, eggs do not mature. Similar inhibitory effects of lowering juvenile hormone were seen in halictine bees and polistine wasps, but not in honey bees.[82]
Other mechanisms
A variety of other mechanisms give queens of different species of social insects a measure of reproductive control over their nest mates. In many
In primitively eusocial bees (where castes are morphologically similar and colonies are small and short-lived), queens frequently nudge their nest mates and then burrow back down into the nest. This draws workers into the lower part of the nest where they may respond to stimuli for cell construction and maintenance.[82] Being nudged by the queen may help to inhibit ovarian development; in addition, the queen eats any eggs laid by workers.[82] Furthermore, temporally discrete production of workers and gynes (actual or potential queens) can cause size dimorphisms between different castes, as size is strongly influenced by the season during which the individual is reared. In many wasps, worker caste is determined by a temporal pattern in which workers precede non-workers of the same generation.[85] In some cases, for example in bumblebees, queen control weakens late in the season, and the ovaries of workers develop.[82] The queen attempts to maintain her dominance by aggressive behavior and by eating worker-laid eggs; her aggression is often directed towards the worker with the greatest ovarian development.[82]
In highly eusocial wasps (where castes are morphologically dissimilar), both the quantity and quality of food are important for caste differentiation.[82] Recent studies in wasps suggest that differential larval nourishment may be the environmental trigger for larval divergence into workers or gynes.[85] All honey bee larvae are initially fed with royal jelly, which is secreted by workers, but normally they are switched over to a diet of pollen and honey as they mature; if their diet is exclusively royal jelly, they grow larger than normal and differentiate into queens. This jelly contains a specific protein, royalactin, which increases body size, promotes ovary development, and shortens the developmental time period.[86] The differential expression in Polistes of larval genes and proteins (also differentially expressed during queen versus caste development in honey bees) indicates that regulatory mechanisms may operate very early in development.[85]
In human culture
Stephen Baxter's 2003 science fiction novel Coalescent imagines a human eusocial organisation founded in ancient Rome, in which most individuals are subject to reproductive repression.[87] Harold Fromm, reviewing Groping for Groups by E. O. Wilson and others in the literary and arts journal The Hudson Review, asks whether Wilson's stated "wish" for "a permanent paradise for human beings", to be brought about by human activity, would mean "to be group-selected in factories in the style of Huxley's [1932 novel] Brave New World.[88]
See also
- Dense heterarchy
- Evolutionarily stable strategy
- International Union for the Study of Social Insects
- Patterns of self-organization in ants
- Reciprocity (social psychology)
- Stigmergy
- Ant colony optimization(ACO)
- Bee colony optimization
- Task allocation and partitioning of social insects
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