Eusociality
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Eusociality is the phenomenon of reproductive specialisation found in some animals. It generally involves the breeding of sterile members of the species, which carry out specialized tasks, effectively caring for the reproductive members.
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 sterile workers.
Evolution of eusocial behavior
There are two causes of eusociality: kin-selected altruism and high inbreeding.
Its defining features are:
- reproductive division of labor
- overlap of generations
- cooperative care of young
- sterile castes (not always present)
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 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, 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 (kin selection or inclusive fitness) had to wait for the discovery of the mechanisms for genetic inheritance.
Eusociality may be easier for species like ants to evolve, due to their haplodiploidy, which increases the significance 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. 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. Even though workers cannot reproduce, they are passing on more of their genes by caring for their sisters than they would by having their own offspring (which would only have 50% of their genes). This unusual situation where females have greater fitness when they are sterile has led to the independent evolution of eusociality occurring 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.
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. In essence, an animal's own red blood cells can be seen as sterile eusocial members of an animal's body colony.
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 animals acting as single, permanent units (slime moulds), to the simplest multicellular life (sponges), from whence all higher animals evolved.
Other examples
Another widespread insect group exhibiting eusociality is the termites (order Isoptera), which in contrast to the Hymenoptera exhibit diploidy. Termites are most closely related to cockroaches and mantids, and are not closely related to the Hymenoptera. The strategy of eusociality arose once in an ancestral termite, whilst it arose several times in the hymenoptera.
Recently, some species of aphids (Order Hemiptera) were found to be eusocial, with many separate origins of the state. This is easily explicable due to their partially asexual mode of reproduction (sterile soldier castes being of the same clone as the reproducing female); most aphids that are related at all are members of the same clone.
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 is also known, in rare cases, among mammals: the naked mole rat is a clear case, and, less rigorously some canids can be argued to be eusocial, since usually only the alpha male and female will breed. In this case, the other members of the pack are not sterile, but suppress breeding in favour of the dominant pair.
Fictional examples
The idea of a human eusocial group was investigated in-depth in Stephen Baxter's novel Coalescent.
The xenomorphs featured in the Alien movies have a eusocial hierarchy, with the Alien Queen breeding the other types.
The Nac Mac Feegle in Terry Pratchett's Discworld series possess a eusocial culture, with hundreds of brothers led by a single mother, or "Kelda".
See also
- Altruism
- Altruism in animals
- Subsociality
- Evolution
- Presociality
- Reciprocity
- Sociobiology
- Superorganism
External links
References
- E.O. Wilson and Bert Hölldobler The Ants, 1990, Belknap Press, ISBN 0-674-04075-9, Winner of the Pulitzer Prize