Everybody knows about evolution by natural selection, mutations and survival of the fittest. It is the mechanism that has made intelligent life possible; without this means of improving efficiency we should still be part of the primeval soup. The mechanism leads to the infinite variety of life; here a variation leading to a better suitability for a particular environment, there another giving one group a competitive edge that leads to dominance, or to elimination of a competitor. It is a complex mechanism, for example temporary isolation of a group can shield it in such a way that it later cannot stand up to others who have been selectively improved by a more competitive situation.
Variety leads to sub-species, leads to species, leads to genus and leads to phylum eventually. In man, sub-race leads to race, leads to sub-species Homo sapiens sapiens, leads to Homo sapiens, leads to genus Homo, and leads to order Primate. These subdivisions usually seem to merge into each other whenever we can see the full story. Biologically there is nothing special about man’s relation to this mechanism except perhaps that Homo sapiens sapiens may be supposed to have dealt particularly ruthlessly with all other competing sub-species and species of Homo, presumably having committed genocide against them with a degree of efficiency derived from his reliance on intelligence as a weapon.
For evolution to occur in any incipient species, which is not living in a physically isolated situation, it is necessary somehow to identify groups of individuals who prefer to interbreed with each other but not with members of related groups; only in this way are they able to perpetuate their particular set of characteristics. This amounts to it being an evolutionary advantage for the group to feel that it is the best group, that its members are the most beautiful, smell good and are generally altogether those with whom the individual prefers to identify himself, at least for the purpose of breeding. The other side of this coin is the feeling that members of a competing group are relatively ugly, smell bad and are rather peculiarly horrible.
In the case of social animals there is, superimposed on this essential mechanism for evolution, another mechanism; that which enables the interbreeding group not only to come into existence with it own characteristics, but for the individuals to carry out cooperative activities as a group. It has therefore become an important built-in element of the evolutionary kit of social animals.