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evolution the theory that describes how all life forms developed from simpler life forms by changes that took millions of years. [1/3 definitions]
microbe A microbe is a tiny life form that can only be seen with a microscope. Many microbes are germs that cause disease.
microorganism any life form so small that it can only be seen with a microscope. Microorganisms include bacteria, viruses, and many protozoans.
mutant a life form whose genes are different from those of its parents. A mutant has new traits or characteristics that it can pass on to its offspring.