This is a very interesting topic. It involves a crucial question about the impact of 'the company culture' in its success or failure. We usually hear that X company has X kind of culture, and the usual advise is something like 'setup the right culture or you will be dead'. However, when I hear 'monoculture' I cannot avoid imagining dystopic landscapes (as in Philip K. Dick's stories or Huxley's Brave New World). Because uniformity kills creativity and is counter-human. But usually, companies try to foster the right attitudes and key values towards work, what, I think, is radically different than a monoculture. In that aspect, the recruitment job is the main filter in order to avoid what you don't want in your company and set the 'right culture'. But also established processes, rules and etiquette set the company's 'culture', besides people's previous bagagge. In the worst scenario, you have no shared culture at all, which may impact in your performance, given the lack of tunning between different teams or within a team. That was my experience in one of the big names of the PC industry. The culture was some kind of random result of inherited practices, new hirings and a bit of wilderness due to the lack of real policies towards a positive culture. And I think this is in some way inherent to big companies, as it is very hard to reach every employee and it's also hard to find so many people that shares their core values (aside of their professional skills). So, in a few words, I just saw different subcultures struggling to make their point of view stand. Which sounds much like 'survival of the fittest'. Even though, I must say, I made there some really true and great friends.
I hope the answer contributes to the discussion.