Social insect

Eusociality is a term used for the highest level of social organization in a hierarchical classification. The term "eusocial" was introduced in 1966 by Suzanne Batra and given a more definitive meaning by E. O. Wilson. It was originally defined to include those organisms (originally, only invertebrates) that had certain features:
 * 1) reproductive division of labor (with or without sterile castes)
 * 2) overlapping generations
 * 3) cooperative care of young

The lower levels of social organization, subsociality, were classified using different terms, including presocial, subsocial, semisocial, parasocial and quasisocial.

Examples
The most familiar examples are insects such as ants, bees, and wasps (the order Hymenoptera), as well as termites (order Isoptera), all with reproductive queens and more or less sterile workers and/or soldiers. The only mammalian examples are the naked mole rat and the damaraland mole rat.

Eusociality with biologically sterile individuals represents the most extreme form of kin altruism. The analysis of eusociality played a key role in the development of theories in sociobiology.

The phenomenon of reproductive specialization is found in various organisms. It generally involves the production of sterile members of the species, which carry out specialized tasks, effectively caring for the reproductive members. It most commonly manifests in the appearance of individuals within a group whose behavior (and sometimes anatomy) is modified for group defense, including self-sacrifice ("altruism").

Definition debates
Subsequent to Wilson's original definition, other authors have sought to expand or narrow the definition of eusociality, focusing on the nature and degree of the division of labor, which was not originally specified. A narrower definition specifies the requirement for irreversibly distinct behavioral groups or castes (with respect to sterility and/or other features), and such a definition excludes all social vertebrates (including mole rats), none of which have irreversible castes. A broader definition allows for any temporary division of labor or non-random distribution of reproductive success to constitute eusociality, and some have accordingly argued that even humans may be considered eusocial. Others believe that the hierarchical classification may not serve much purpose.

Evolution of eusociality
Early ideas on eusociality included suggestions that trophallaxis or food sharing was a basis for sociality. Other theories include superorganism theory and parental manipulation theory. The most widely accepted model to explain eusociality is based on the idea of inclusive fitness.

According to inclusive fitness theory, eusociality may be easier for species like ants to evolve, due to their haplodiploidy, which facilitates the operation of kin selection. Sisters are more related to each other than to their offspring. This mechanism of sex determination gives rise to what W. D. Hamilton first termed "supersisters" who share 75 per cent of their genes on average. Sterile workers are more closely related to their supersisters than to any offspring they might have, if they were to breed themselves. From the "selfish gene's" point-of-view, it is advantageous to raise more sisters, assuming that roughly equal numbers are being produced. Even though workers often do not reproduce, they are potentially passing on more of their genes by caring for their sisters than they would 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 siblings rather than producing offspring is often invoked to explain the multiple independent evolutions of eusociality (occurring some 11 separate times) within the haplodiploid group Hymenoptera — ants, bees and wasps. However, Hymenoptera is a large group and the majority of hymenopterans are not social. Furthermore, highly developed eusociality also exists in non-hymenopterans, perhaps most prominently in termites. Certain vertebrates (such as the naked mole rat) have also been described as eusocial. Most such cases involve organisms that display high levels of inbreeding, such that colony members share more than 50% of their genes, and therefore the same model is considered to apply to these species.

Superorganism theory, in contrast, explains the evolutionary stability of eusociality by focusing on competition among groups of organisms, such that selection upon features of the behavior of the group as a whole outweighs selection on the individuals within each group; that is, there is a higher payoff for an individual to invest in between-group competition than to invest in within-group competition. Support for this is based on the observation that multicellular life essentially started out as colonies of one-celled creatures, in which most of the one-celled creatures became specialized to other roles in the colony, losing the ability to reproduce. Thus came the transition from hordes of cooperating one-celled animals (algae are an example) to colonies of one-celled organisms acting as single, permanent units (slime moulds), to the simplest multicellular life (sponges), from which all higher animals evolved.

Theories of parental manipulation point out that the transition from solitary to eusocial appears to involve intermediate stages where dominance interactions are required to suppress the reproductive tendencies of group members; that is, females are manipulated into acting as workers, even if it is against their own self-interest. This model does not require that individuals be highly related, though high relatedness will reduce expected levels of resistance to manipulation.

In spite of the obvious advantages of common foraging and defense, eusocial animals had appeared paradoxical even to Darwin: if adaptive evolution unfolds by differential survival of successful species, how can a species succeed in which most individuals don't breed at all? How can individuals incapable of passing on their genes possibly evolve and persist? Since they do not breed, their fitness should be zero and any alleles causing this condition should be eliminated from the population immediately. In Origin of Species (first edition, Ch. 7), Darwin called this behavior the "one special difficulty, which at first appeared to me insuperable, and actually fatal to my theory." Darwin anticipated that the resolution to the paradox would lie in the close family relationship, but specific theories to offer such resolution (e.g., kin selection or inclusive fitness) had to wait for the discovery of the mechanisms for genetic inheritance.

Some hypotheses about how eusociality evolved in naked mole rats include: inbreeding, ecological factors such as the dependence on large tubers which are hard to locate and reach underground, heat loss prevention, and high dispersal costs. In the mammalian cases, eusociality is believed to arise from 'reproductive suppression', where infertility in working females is only temporary, and not genetic.

Other examples
Another widespread insect group exhibiting eusociality is the termites (order Isoptera), which in contrast to the Hymenoptera exhibit diploidy, like most organisms. Termites are a lineage of cockroaches, and are not closely related to the Hymenoptera. Eusociality arose once in an ancestral termite, whilst it arose several times in the Hymenoptera.

Recently, some species of gall-making aphids (Order Hemiptera) and thrips (Order Thysanoptera) were found to be eusocial, with many separate origins of the state. These species have extremely high relatedness among individuals due to their partially asexual mode of reproduction (sterile soldier castes being of the same clone as the reproducing female), but the gall-inhabiting behavior gives these species a defensible resource that sets them apart from related species with similar genetics. In these groups, therefore, high relatedness alone does not lead to the evolution of social behavior, but requires that groups occur in a restricted, shared area.

Similarly, eusociality has arisen among some crustaceans and other arthropods. On some tropical reefs, several species of minute Synalpheus pistol shrimp that depend on certain sponges for the survival of their colony, live eusocially, with a single breeding female and a preponderance of male defenders, armed with enlarged snapping claws. Again, there is a single shared domicile for the colony members, and the non-breeding members act to defend it.