The Ethological Explanation of Aggression, including reference to Innate Releasing Mechanisms and Fixed Action Patterns. Evolutionary Explanations of Human Aggression.

Lorenz (an Evolutionary Psychologist) observed that fights between animals of the same species resulted in little actual physical damage. Most aggressive encounters comprised of mainly ritualistic signalling (e.g. displaying claws, baring of teeth etc) and rarely reached the point of becoming physical. Typically, intra-species aggressive encounters end with ritual appeasement displays. These indicate acceptance of defeat and inhibit aggressive behaviour in the victor, preventing damage to the loser (e.g. in defeat a wolf will expose his neck to the victor, deliberately making itself vulnerable).

Genetic factors in aggression, including the MAOA gene

Several studies have suggested that heritability accounts for about 50% of the variance in aggressive behaviour. Monozygotic (identical) twins share 100% of their genes, while dizygotic (non-identical) twins share only about 50%, as a result we would expect to find greater similarities in aggressive behaviour between MZ twins if aggression is mostly influenced by genetic factors. Examples of concordance rates for different types of aggression are below.
Physical aggression MZ=50% and DZ=19%, Verbal aggression MZ=28% and DZ=7%.