Natural selection at the group level: The myxoma virus was introduced into rabbit populations in Australia in the 1920's to control the overwhelming rise in rabbit numbers. To the surprise of the researchers, the forms of the virus most "successful" (that is, which had infected the most hosts) were not the most virulent forms. In fact the most successful version of the virus was rather more mild because the extremely effective form of the virus killed the host before transmission was possible. The less virulent form, slower to kill the host, better exposed potential hosts to which the virus could be transferred. This is an example of natural selection at the group level e.g., the more "fit" (lethal) virus went extinct because it could not pass itself on.
Kin selection: a member of a lemur group signals the approach of a predator. In calling out to its fellow lemurs it makes the predator aware of its location and is subsequently eaten. The one organism sacrifices its own life -- thereby forfeiting its opportunity to breed -- and yet its siblings, which carry much of the same genome, persist to create offspring. The loss of one whose genes are largely possessed by others is a selective force for the genome from which that individual arose insofar as the others continue because of the sacrifice.
If we are to reduce the cooperation of monks to natural selection, I would suggest that it is kin selection that is at work. That is to say, we are brothers and sisters committed to a common goal. Here it is not survival, but the proliferation of an idea. If one (e.g., tilly) falls, we heed the warning* and regroup ourselves to push forward. In this case it is not about our genes but rather about the purpose that we endeavor to fulfill.
* Of course when an animal indicates to its group it is an indexical "statement", that is, equivalent to a call meaning i.e., "danger here now" or "food here now", whereas we are capable of far more abstract communications.