Before the mid-1800s, classification of organisms into groups, called taxa, was generally based on overall similarity of physical appearance. There was no guiding principle as to why the members of one group were more similar to each other than to the members of other groups. In 1859, Charles Darwin's Origin of Species was published, and Darwin's theory of evolution provided the explanation that natural groups occur because the members of the group are the descendants of a common ancestor. Based on Darwin's principles, in 1866, the German naturalist, Ernst Haeckel, coined the term phylogeny to describe the "science of the changes in form through which the phyla or organic lineages pass through the entire time of their discrete existence." Today the term phylogeny is used more widely to mean the evolutionary history or exact genealogy of a species or group of organisms. Phylogenies are based on the study of...