We present experimental results on the ultimatum bargaining game which support an evolutionary explanation of subjects' behaviour in the game. In these experiments subjects interacted with each other and also with virtual players, i.e. computer programs with prospecified strategies. Some of these virtual players were designed to play the equitable allocation, while others exhibited behaviour closer to the subgame-perfect equilibrium, in which the proposer's share is much larger than that of the responder. We have observed significant differences in the behaviour of real subjects depending on the type of "mutants" (virtual players) that were present in their environment.