Protists are a diverse group of organisms, and they reproduce in a number of different ways, including asexual binary fission, multiple fission, fragmentation and several forms of sexual reproduction. Many protists can reproduce either sexually or asexually depending on environmental conditions.
Protists are subdivided into four groups: protozoa, unicellular algae, slime molds and water molds.
All are eukaryotic, meaning their cells contain a nucleus enclosed by a membrane. While they are classified separately, scientists believe the various types of protists paved the way for the evolution of early plants, animals and fungi. Eukaryotic organisms first appeared more than 1.5 billion years ago. Scientists believe the first eukaryotes were protists, and that they reproduced asexually. The simplest form of asexual reproduction is binary fission, where a single DNA molecule replicates and the original cell divides into two daughter cells, each with a copy of the original DNA. Multiple fission occurs by budding or by the production of spores and results in a number of daughter cells. Asexual reproduction produces daughter cells that are genetically identical to the parent. Over the course of time, some protists developed the capability of reproducing sexually. In sexual reproduction, a parent nucleus divides by meiosis to produce gametes, which have only half the DNA of the parent. When gametes from two parent cells fuse, the result is an offspring with a complete and novel set of genetic material. The genetic mixing achieved by sexual reproduction ensures genetic diversity, which is advantageous when adapting to changing environmental conditions.