Protists reproduce in many ways depending on their environment, life cycle, and type, either asexually—by budding, binary fission, or mitosis—or sexually. During binary fission, the organism’s DNA replicates and the cells divide; during budding, the organism produces a smaller bud of itself that will grow into an individual protist identical to the original.

Some protists use a variety of reproductive methods; for example, paramecium reproduce using binary fission, but after so many hundreds of times essentially splitting apart, the paramecium use sexual reproduction to exchange their genetic material. Scientists do not know what triggers this sudden urge on the protist’s part to sexually reproduce—but it may have to do with surrounding environmental stresses. (For more about the environment see the chapter “Environment and Ecology.”)