The theory of evolution requires large and fundamental changes in organisms over time due to natural selection over diverse populations with characteristics being passed on by reproduction. Such that very different organisms can have common reproductive ancestors.
It does not in fact require a specific number of clearly delineated categories of organisms where all individual organisms are in one and only one category.
That part is a human invention to try to make sense of the world, rather than part of the mechanics of the world itself. The categorization is probably a necessary invention to try to make sense of the world in a scientific systematic way, and is also inevitably a broken inconsistent subjective biased over-simplified map of the territory.
It does not in fact require a specific number of clearly delineated categories of organisms where all individual organisms are in one and only one category.
That part is a human invention to try to make sense of the world, rather than part of the mechanics of the world itself. The categorization is probably a necessary invention to try to make sense of the world in a scientific systematic way, and is also inevitably a broken inconsistent subjective biased over-simplified map of the territory.