Multicellular organisms begin as a single cell. These cells then grow and undergo differentiation, the process by which cells develop specialized forms and functions.In multicellular organisms, cells are often organized into tissues, organs, and organ systems.
If you are referring to selection pressure, when the selection pressure decreases, there will be weaker forces of natural selection. The angler fishes without the favourable traits would not be that strongly selected against and vice versa. In some cases such as predation selection pressure, the population of angler fishes in the habitat may increase
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