See http://en.wikipedia.org/wiki/Hundredth_Monkey for background. The story is apocryphal, a myth. But maybe something like that effect happens sometimes in evolution of species or of cultures. If it does happen, it could either be a speciation event or just a fixation of a mutant gene/meme within a species/culture. The part that is controversial is a proposed wholistic, global effect that does not involve local transfer of memes from monkey to monkey (or person to person). Something like collapse of a wave function or morphic fields. There is no data from evolutionary biology to support such a scenario, as far as I know, and I won't be discussing this scenario, as interesting as it might be. I will however discuss a way that critical population size could play a role in speciation or fixation of a meme/gene. Before we discuss how something like a critical mass could be essential for speciation (or for the fixation of a mutated gene/meme), we need to review 3 possible speciation scenarios.
1. The mountain pass scenario, when a reproductively isolated population needs to overcome a fitness barrier (mountain pass and think of negative fitness for the analogy to work) in order to then descend into a new valley where it can optimize its fitness and become a new species (splitting off from the parent species).
2. The tunneling scenario, when a reproductively isolated population has a hidden phenotype due to a genetic/memetic mutation and emerges into the other valley when it has traveled far enough in geographical and/or genetic/memetic space, without a fitness penalty (splitting off from the parent species).
3. The chronospeciation/entropic barrier scenario, when the whole population makes the shift (no reproductive isolation), going "downhill" (in negative fitness) all the way, finding a rare downhill path among mostly uphill paths. The downhill path may have been there for a while but was not found, or just emerged to to changing environment.
Keeping these speciation scenarios in mind, there are three posssible ways that a hundredth monkey scenario could operate:
1. When there is no speciation, but only fixation of a gene/meme, the mutated sub-population has reached a critical mass where the probability of being swamped out by genetic drift from the unmutated population is very small, and so the gene/meme almost certainly gets fixated in the population at large.
2. When there is reproductive isolation, a mutation in a master/regulatory meme, and a mountain pass or tunneling scenario, the small new species becomes large enough to be stable and not die out. This is once they have gone over the mountain pass or tunnelled through the mountain. In a mountain pass scenario, there is the possibility of a critical mass being necessary even while going up to the pass, because more people who are already manifesting some of the new culture might actually be able to lower the barrier or be able to handle and thwart the old culture manifesting in new people. I feel like this is the situation with my project (Open Space Church or Arkwright).
3. Perhaps a critical mass could be important in a chronospeciation event (no splitting into two species) without reproductive isolation in a scenario with entropic barriers. A few individuals go down the new path and thrive, but no one follows until a critical mass is reached because people (or monkeys) from the old species/culture are not going to notice the thriving people/monkeys in the new species or perhaps they notice and persecute or make it hard for those monkeys/people, being conservative in nature and apt to believe that the old ways are better. I think this might be the situation for the Possibility Alliance.