I described Dawkins's ''weasel algorithm'' in a previous posts. Some claim that in the process the correct letters are ''explicitly or implicitly'' latched. That's not true, the algorithm isn't necessarily monotonously nondecreasing: a change to the worse from one generation to the next at least once while running the program is possible.
I calculated the probabilities for two case: a population of ten and a population of fifty  both times with a mutation rate of 4%.
10: 95.7 %
50: 0.0000026 %
With a small population, the effect will most probably be observed. But even with the bigger population, a programmer may spot the event  if he is lucky enough.
Why did I chose this population? A population of ten seems to be used by R. Dawkins in his video of 1987, while a population size of fifty was probably used for his book in 1986.
In the first case, it takes 1306 generations on average (σ=924), in the second case, 139 generations (σ=48). So it is quite probable that observe such a reversal, as seen on the video, two. For the bigger population such an occurence is at least more likely than winning a lottery....
Here is a complete view for various rates of mutation ( 1%  10%) and populations from 1 to 100:
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