Character evolution is the process by which a character or trait (a certain body part or property of an organism) evolves along the branches of an evolutionary tree. Character evolution usually refers to single changes within a lineage that make this lineage unique from others. These changes are called character state changes and they are often used in the study of evolution to provide a record of common ancestry. Character state changes can be phenotypic changes, nucleotide substitutions, or amino acid substitutions. These small changes in a species can be identifying features of when exactly a new lineage diverged from an old one.
In the study of phylogenetics or cladistics, researchers can look at the characters shared by a collection of species and then group them into what is called a clade. The term clade was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis, a concept Huxley borrowed from Bernhard Rensch. A clade is by definition monophyletic, meaning it contains one ancestor (which can be an organism, a population, or a species) and all its descendants.
It is often the case in the study of phylogenies that the vast majority of organisms of interest are long extinct. It is therefore a matter of speculation to reconstruct what ancestral organisms existed long before the present time, and how the evolutionary process led from one organism to another, and which present-day organisms are most closely related. Character evolution and the character state changes that drive this type of evolution are what help researchers construct these trees in a fashion referred to as maximum parsimony. When talking about phylogenetics, maximum parsimony refers to a method of inferring a phylogenetic tree in a way that minimizes the number of implied character state transformations in the observed data (hence maximally parsimonious). The basic ideas were presented by James S. Farris in 1970.