Eusociality is the phenomenon of reproductive specialisation found in some species of animal. Its defining features are:

  1. cooperative parental care
  2. overlap of generations
  3. sterile castes

The most familiar examples are insects such as ants, bees, and wasps (the order Hymenoptera), with reproductive queens and sterile workers. Eusociality may be easier for these species to evolve due to their haplodiploidy, which increases the significance of kin selection. This complex mechanism of sex determination gives rise to what W. D. Hamilton first termed "supersisters" who share 75 per cent of their genes. 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, there is a genetic advantage to raising more sisters. However, this has been proven to not be as convincing as it seems, as this also leads to skewing of the sex ratio towards females, and a stable sex ratio of 3:1 females to males. As a result, males become 3 times as fit as females due to the extra matings they receive due to being in short supply. Factoring in this, haplodiploidy does not seem as likely to predispose a species to eusociality any more than diploidy, although this depends on if the females being counted are physically sterile or not.

As a result of this, it has been inferred that the evolution of eusociality from haplodiploidy must involve the skewing of the sex ratio towards females, without the subsequent loss of fitness. Current theories as to how this occur have cited Partially bivoltine insects such as the Hallictine Bees and Sphecid wasps.

Superorganism theory explains the evolutionary stability of eusociality by dictating that only reproductive individuals are counted as individuals and sterile individuals are simply independent parts of their reproductive parent. This theory makes sense only when the sterile caste is physically sterile and not simply being repressed. In this way the sterile caste provide for their reproductive parents so that their genes can spread through them.

It is thought that eusociality evolved 11 separate times within the order of Hymenoptera. The Hymenoptera are often referred to as "social insects," but it is now preferred to use the term "eusocial insects" for those species that exhibit eusociality, and not to apply the term "social insects" to other Hymenoptera; it is important to realise that not all the Hymenoptera are eusocial.

Another extremely widespread insect group exhibiting eusociality are the termites (order Isoptera), although it is not well known how eusociality could evolve in these species, since they exhibit diploidy. Recently, some species of aphids (Order Hemiptera) were found to be eusocial, which is easier explained due to their partially asexual mode of reproduction (sterile soldier castes being of the same clone as the reproducing female). Eusociality is also known among mammals: the naked mole rat and the Delicate Slender Opossum are clear cases, and, less rigorously some canids can be argued to be eusocial, since only the alpha male and female will breed. In this case, the other members of the pack are not sterile, but are dissuaded from breeding by aggressive behavior on the part of the breeding pair.

Eusociality has arisen among some crustaceans and other arthropods. On some tropical reefs, several species of minute synalpheid 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.

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

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? In Origin of Species, Darwin called this altruistic 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 the complete answer had to wait for the discovery of the mechanisms for genetic inheritance.

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