Co-operation or co-operative behaviours are terms used to describe behaviours by organisms which are beneficial to other organisms, and are selected for on that basis. Under this definition, altruism is a form of co-operation in which there is no direct benefit to the actor (the organism carrying out the behaviour). Co-operative behaviour in which there is a direct benefit to the actor as well as the recipient can be termed "mutually beneficial". There are several theories which help to explain why natural selection favours some types of co-operative behaviour. They are not mutually exclusive, however, and more than one of the theories discussed below may contribute to explaining a particular case of co-operative behaviour.
One well accepted explanation for altruistic behaviour (that is, co-operative behaviour which lacks a direct benefit for the actor) is the theory of kin selection. This theory suggests that individuals act co-operatively in order to help others which are genetically similar. Genes for such co-operative behaviour are preserved, because they help to perpetuate their own existence. The classic example is the social insects, such as bees and ants. Worker insects never reproduce, but instead, they work to allow the (genetically similar) queen to reproduce.
The theory of reciprocity suggests that individuals carry out co-operative behaviours because they get something in return. In order for such behaviours to be favoured, there needs to be some perception of external physical markers that the other individual will recognise (otherwise, there is no selective pressure to maintain the behaviour). Much research about reciprocity as leading to co-operation has concentrated on the 'prisoner's dilemma' known from game theory.
One theory suggesting a mechanism that could lead to the evolution of co-operation is the "market effect" as suggested by Noe and Hammerstein. The mechanism relies on the fact that in many situations there exists a trade-off between efficiency obtaining a desired resource and the amount of resources one can actively obtain. In that case, each partner in a system could benefit from specializing in producing one specific resource and obtaining the other resource by trade. When only two partners exist, each can specialize in one resource, and trade for the other. Trading for the resource requires co-operation with the other partner and includes a process of bidding and bargaining.
This mechanism can be relied to both within a species or social group and within species systems. It can also be applied to a multi-partner system, in which the owner of a resource has the power to choose its co-operation partner. This model can be applied in natural systems (examples exist in the world of apes, cleaner fish, and more). Easy for exemplifying, though, are systems from international trading. Arabic countries control vast amounts of oil, but seek technologies from western countries. These in turn are in need of Arab oil. The solution is co-operation by trade.
Multi-level selection theory suggests that selection operates on more than one level: for example, it may operate at an atomic and molecular level in cells, at the level of cells in the body, and then again at the whole organism level, and the community level, and the species level. Any level which is not competitive with others of the same level will be eliminated, even if the level below is highly competitive. A classic example is that of genes which prevent cancer. Cancer cells divide uncontrollably, and at the cellular level, they are very successful, because they are (in the short term) reproducing very well and out competing other cells in the body. However, at the whole organism level, cancer is often fatal, and so may prevent reproduction. Therefore, changes to the genome which prevent cancer (for example, by causing damaged cells to act co-operatively by destroying themselves) are favoured. Multi-level selection theory contends that similar effects can occur, for example, to cause individuals to co-operate to avoid behaviours which favour themselves short-term, but destroy the community (and their descendants) long term.
- Nowak MA (2006). Five rules for the evolution of cooperation. Science 314: 1560-1563. 
- Mikhail Burtsev and Peter Turchin: Evolution of cooperative strategies from first principles, Nature 440, 1041-1044 (20 April 2006)
- West, S.A., Griffin, A.S., and Gardner, A. (2007) Evolutionary explanations for cooperation. Current Biology 17: R661-R672. 
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