What is Phylogeny?

Phylogeny is the evolutionary history of an organism. The great 19th century naturalist Ernst Haeckel coined the word phylogeny. It comes from Greek words roughly meaning "birth of races." Phylogeny is like the genealogy of living things, and a phylogeny tree is like a family tree.

Phylogeny can refer to the evolutionary history of an individual, a population, or a group of populations. It can even be traced for an individual gene because genes are the basic unit of evolution.

Phylogeny is an area of active research by biologists. Cladistics, a systematic way of tracing phylogeny based on shared characteristics of organisms, is the preferred discipline used to study phylogeny. With the advent of DNA sequencing technology, researchers using cladistics have another detail-rich tool for tracing the phylogeny of living things.

In the history of science, the ideas of Plato, who thought that every thing has an unchangeable "essence," influenced natural philosophy until the Scientific Revolution. If there is no change, there is no evolution, and therefore no evolutionary history. When the first theories of materialistic evolution were proposed around the 18th century, naturalists began to describe phylogeny based on their observations of nature.

An artist, Haeckel drew the Tree of Life as a literal tree, complete with bark and roots. His phylogeny proposed some relationships that seem bizarre today; for example, he placed birds and turtles as sister groups on a branch separate from other reptiles. Every modern phylogeny tree places turtles as the most basal branch of reptiles.

Haeckel's famous phylogeny tree is titled "The Pedigree of Man." Its design places different forms of life into ranks with humans as the pinnacle of evolution. The biggest conceptual distinction between this idea of phylogeny and those of modern science is that the idea of rank has been dismissed. Phylogeny trees no longer refer to higher and lower animals. The only axis used now is time.

To account for this change in concept, cladistics researchers have developed a new vocabulary. Instead of "lower" or "primitive," they prefer the term "basal," referring to a branch whose origin is relatively close to the base of a phylogeny tree. Instead of "higher" or "advanced," they use "derived," meaning something that has been derived through evolution from a more basal form.

In a modern phylogeny tree, with time as the only axis, all non-extinct species are placed at the same height. A phylogeny tree that includes extinct, fossilized species often shows those species' branches as ending at the point in geological time when they went extinct. Since rank has been eliminated, a phylogeny tree may be shown in circular rather than linear form to save space.

Phylogeny is an important concept in biology. Taken as a concept from evolution, it is used for classification and for understanding the nature and history of life.