Biology

If you have an environment where organisms are able to leave but not enter (this is applicable to bacteria in animal intestines, just take my word for it), then you have a fairly unique situation. The population of that environment can expand only by reproduction, but it can be reduced either by death of the organisms or by their removal from the ecosystem. In that case the selection pressure to stay in that environment and the selection pressure to survive are equal. Depending on your beliefs you can look at Earth as such an environment with some other environment beyond, and death and movement to the beyond environment are biologically equal. Maybe intestinal bacteria would think about these things if they could. Dude.