Frank Harlan Lewis, known professionally as Harlan Lewis, was an American botanist, geneticist, taxonomist, systematist, and evolutionist who worked primarily with plants in the genus Clarkia. He is best known for his theories of "catastrophic selection" and "saltational speciation", which are closely aligned with the concepts of quantum evolution and sympatric speciation. The concepts were first articulated in 1958 by Lewis and Peter H. Raven, and later refined in a 1962 paper by Lewis in which he coined the term "catastrophic selection". In 1966, he referred to the same mechanism as "saltational speciation".
Lewis was Dean of Life Sciences at the University of California, Los Angeles from 1962 to 1981, a Fellow of the American Association for the Advancement of Science, recipient of a Guggenheim Fellowship, president of the Pacific Division of the Botanical Society of America, president of the Society for the Study of Evolution, president of the American Society of Plant Taxonomists, president of the International Organization of Plant Biosystematics, and president of the American Society of Naturalists, as well as a Fellow of the California Academy of Sciences.