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Tribalism and evolution
Tribalism has a very adaptive effect in human evolution. Humans are social animals, and ill-equipped to live on their own. Tribalism and social bonding help to keep individuals committed to the group, even when personal relations may fray. This keeps individuals from wandering off or joining other groups. It also leads to bullying when a tribal member is unwilling to conform to the politics of the collective.
Socially, divisions between groups fosters specialized interactions with others, based on association: altruism (positive interactions with unrelated members), kin-selectivity (positive interactions with related members), and violence (negative interactions). Thus, groups with a strong sense of unity and identity can benefit from kin selection behavior such as common property and shared resources. The tendency of members to unite against an outside tribe and the ability to act violently and prejudicially against that outside tribe likely boosted the chances of survival in genocidal conflicts.
Modern examples of tribal genocide rarely reflect the defining characteristics of tribes existing prior to the Neolithic Revolution--for example, small population and close-relatedness.
According to a study by Robin Dunbar at the University of Liverpool, primate brain size is determined by social group size. Dunbar's conclusion was that most human brains can only really understand an average of 150 individuals as fully developed, complex people (Known as Dunbar's number). In contrast, anthropologist H. Russell Bernard and Peter Killworth have done a variety of field studies in the United States that came up with an estimated mean number of ties, 290, that is roughly double Dunbar's estimate. The Bernard–Killworth median of 231 is lower, due to upward straggle in the distribution, but still appreciably larger than Dunbar's estimate. 
Malcolm Gladwell expanded on this conclusion sociologically in his book, The Tipping Point where one of his types - Connectors - were successful due to their larger than average number of close friendships and capacity for maintaining them which tie otherwise unconnected social groups together. According to these studies, then, "tribalism" is in some sense an inescapable fact of human neurology, simply because many human brains are not adapted to working with large populations. Once a person's limit for connection is reached, the human brain must resort to some combination of hierarchical schemes, stereotypes, and other simplified models in order to understand so many people.
Nevertheless, complex societies (and corporations) rely upon the tribal instincts of their members for their organization and survival. For example, a representative democracy relies on the ability of a "tribe" of representatives to organize and deal with the problems of an entire nation. The instincts that these representatives are using to deal with national problems have been highly developed in the long course of human evolution on a small tribal scale, and this is the source of both their usefulness and their disutility. Indeed, much of the political tension in modern societies is the conflict between the desire to organize a nation-state using the tribal values of egalitarianism and unity and the simple fact that large societies are unavoidably impersonal and sometimes not amenable to small-society rules.
In complex societies, this tribalistic impulse can also be channelled into more frivolous avenues, manifesting itself in sports rivalries and other such "fan" affiliations.