Walter Monroe Fitch (May 21, 1929 - March 10, 2011) was a pioneering American researcher in molecular evolution.
Fitch spent 24 years at the University of Wisconsin-Madison, followed by three years at the University of Southern California and then was professor of molecular evolution at the University of California, Irvine, until his death. He was a member of the National Academy of Sciences, the American Philosophical Society, and the American Association for the Advancement of Science, and was a Foreign Member of the London Linnean Society. He co-founded the journal Molecular Biology and Evolution, with Masatoshi Nei, and was the first president of the Society for Molecular Biology and Evolution.
Fitch is noted for his pioneering work on reconstruction of phylogenies (evolutionary trees) from protein and DNA sequences. Among his achievements are the first major paper on distance matrix methods, which introduced the Fitch-Margoliash method (with Emanuel Margoliash) which seeks the tree that best predicts a set of pairwise distances among species. He also developed the Fitch maximum parsimony algorithm, which evaluates rapidly and exactly the minimum number of changes of state of a sequence on a given phylogeny. His definition of orthologous sequences has been frequently cited and is used as a reference in many research publications.