Each generation, I am saving the best strategies to run again in the next generation. There is no limit to how long a strategy can be kept - as long as it doesn't get eliminated, it will be kept.
However many strategies are eliminated, I will create the same number of new strategies, so that in each round of play there are the same number of strategies, 100, competing. Each new strategy is created by combining two existing strategies. These two strategies are picked at random from the strategies that scored well enough to be kept for the next generation.
Now I am sorting the scores on each round of play, so I can control the fraction of strategies I eliminate. Here is the graph of evolution of the probability of unprovoked defection, when I eliminate the bottom scoring half of the strategies in each generation:I have added a couple new lines to this graph - now I am plotting the smallest and the largest values across the 100 strategies, along with the 20th and 80th percentiles, and the median.
If instead I only eliminate the bottom scoring 20% of the strategies, here is how things evolve: