In the evolutionary approach to the repeated prisoner's dilemma, strategies spread in populations of emulating and experimenting agents through the principle of survival of the fittest. Although no pure strategy is evolutionarily stable in such populations, the processes of differential strategy propagation provide a promising area of study. This paper employs computer simulations to uncover how these processes govern the oscillating and open-ended evolution of alternative forms of behaviour. Certain `ecological' relationships between important strategy types which are found to be responsible for these behavioural shifts are explored.