symbiotic relationship is 2 different organisms living together in close proximity with one providing essential material or service to the other. its evolution is complicated n takes a very long time. the relationship may start out by random chance of 2 organisms get close together. as the relationship provided benefits to 1 or both organisms, it gives 'em an advantage over others which do not have such relationship. over time, the given advantage drive 2 organisms together n eventually the symbiotic relationship is established.
So, I read up on this just to answer this question. In basic terms, it revolves around natural selection. Natural selection is simply when the organisms that are 'better adapted' live on to pass on their genes. The animals that aren't the 'better adapted' have to find ways to survive. This is what leads to the development of these relationships. For example, small insects may climb onto animals to get food. The organisms support each other. Does that help?
