Phylogeny

Phylogeny is the history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms.
Phylogeny proposes that plants or animals of different species descended from common ancestors. The are a huge number of species that have ever lived are extinct, and relatively few of their remains have been preserved in the fossil record. Most phylogenies therefore are hypotheses and are based on indirect evidence. There is universal understanding that the tree of life is the result of organic descent from earlier ancestors and that true phylogenies are discoverable.

Evidence for specific phylogenies

Most of the data used in making phylogenetic judgments have come from comparative anatomy and from embryology, although these are rapidly being surpassed by systems constructed using molecular data. In comparing features common to different species, anatomists try to distinguish between homologies, or similarities inherited from a common ancestor, and analogies, or similarities that arise in response to similar habits and living conditions.


By counting differences in the sequence of units that make up protein and deoxyribonucleic acid (DNA) molecules, researchers have devised a tool for measuring the degree to which different species have diverged since evolving from a common ancestor. Because mitochondrial DNA has very high mutation rates compared with nuclear DNA, it has been useful for establishing relationships among groups that have diverged recently. Essentially, the application of molecular genetics to systematics is similar to the use of radioisotopes in geologic dating: molecules change at different rates, with some, such as mitochondrial DNA, evolving rapidly and others, such as ribosomal RNA, evolving slowly. An important assumption then in using molecules for phylogeny reconstruction is to select the appropriate gene for the age of the taxon under study.