Would you say cellular endosymbiosis theory opposes that of natural selection?
Definitely not! Cellular endosymbiosis explains the existence of more complex cell types. A smaller prokaryotic cell with the ability to break down certain compounds for energy would have had to compete with others of its type for space/to avoid being eaten by predatory prokaryotes. Living inside of a much larger prokaryote solved these problems, and gave the small prokaryote an advantage over others of its type – it would have been selected for. The larger prokaryote also gained a new energy source from its tag-along friend, which would have given it an advantage over other members of its “species”, too. Edit: Ah, I see what you mean more now. Natural selection does not have to mean changes only in one particular organism – it only means that organisms with advantages over the others tend to survive more and have more offspring (this is what is means when we say an organism has been “selected for”). There are lots of examples of co-evolution – species evolving together – to be found i
