Eusociality Totally Explained

Eusociality is the phenomenon of reproductive specialization found in some animals. 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").
   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 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.
   The most commonly accepted defining features of eusociality are:

reproductive division of labor (with or without sterile castes)
overlap of generations
cooperative care of young (including protection)

Evolution of eusocial behavior

There are several phenomena and theories which have been commonly invoked to explain the evolution of eusociality. The most prominent theory involves genetics: inclusive fitness, also known as kin-selected altruism. Other theories include superorganism theory and parental manipulation theory.
   In spite of the obvious advantages of common foraging and defense, eusocial animals present a seeming paradox, which troubled 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 don't 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 (for example, kin selection or inclusive fitness) had to wait for the discovery of the mechanisms for genetic inheritance.
   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's advantageous to raise more sisters. Even though workers don't reproduce, they're passing on more of their genes by caring for their sisters than they'd by having their own offspring (which would only have 50% of their genes). This unusual situation where females 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's a higher payoff for an individual to invest in between-group competition than to invest in within-group competition.
   Indeed, multicellular life seems to have 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's against their own self-interest. This model doesn't require that individuals be highly related, though high relatedness will reduce expected levels of resistance to manipulation.

Other examples

Another widespread insect group exhibiting eusociality is the termites (order Isoptera), which in contrast to the Hymenoptera exhibit diploidy. 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 doesn't 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 outsize snapping claws. Again, there's a single shared domicile for the colony members, and the non-breeding members act to defend it.
   Eusociality is also known, in two cases, among mammals: the naked mole rat and the damaraland mole rat. 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.

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