Cells, tissue, organs, organ systems, and organisms.
I believe it is behavioural isolation. When there are changes within a certain species, e.g. new birdcalls are formed in certain individuals in a species, "traditional" birds may not like such mating calls and only choose to mate with the more common birdcalls from majority of the birds. There are also some birds that may like the new birdcalls, and choose to mate with birds with those birdcalls. They become genetically isolated as they continue such behavior. Eventually, through genetic drift and other mutations, different species form from a single specie.
Hope this helps! :)
This seems rather vague, the question is most likely referring to parasitic relationships.<span />
If waves continuously crash down on the land over time, the shore begins to erode because of the amount of pressure constantly pushes down on it, which can cause the shore to eventually wear down.
