Phylogenetics: Definition & Analysis

Instructor: Duane Cloud

Duane has taught teacher education courses and has a Doctorate in curriculum and instruction. His doctoral dissertation is on ''The Wizard of Oz''.

Discussions on the diversity of life often rely on a large diagram known as the Tree of Life. This kind of diagram shows the evolutionary path taken by organisms. How these organisms are classified into such a diagram is the field of phylogenetics.

Phylogenetics

Ever wonder where scientists get those odd Latin names for things? Why would someone use a Latin term to describe something? Isn't Latin a dead language? The answers to these questions are part of a field called phylogenetics. Put simply, this field studies the traits of organisms and their kinship to one another. For more information on phylogeny, read on.

Definition and Classification

Phylogeny is the method by which organisms are classified via morphology (physical form and structure) and molecular data (DNA and similar proteins). Phylogenetics is a newer form of classification than the traditional Linnaean classification system. The idea behind modern phylogeny is to record organisms in such a way that each one has a place indicated by evidence of common ancestry through evolution. The phylogenetic system depends upon classifying organisms into a double-nested hierarchy. This is essentially a tree-like diagram where every branch describes a single characteristic and whether or not a particular organism possesses it.

The Linnaean system was formulated by Carl Linnaeus, who classified creatures by their similarities to one another. The newer system, phylogenetics, uses a similar approach, but also depends upon data from DNA and other molecular sources to develop a picture of organisms' ancestries and their relationships to other organisms. Phylogeny still closely resembles Linnaeus's system, and most organisms are in similar places - and thus show similar relationships - in both systems.

One major exception to this is reptiles, which formed a valid classification under Linnaeus's system but not under modern phylogeny. Since it was discovered that birds shared a common ancestor with dinosaurs, reptiles under modern phylogeny would include birds as the only extant (non-extinct) branch of dinosauria. Other reptiles are classed according to their own nearest ancestors, such as crocodiles (crocodilomorpha) and snakes (squamata). The modern scientist is offered a choice: include birds in the classification as reptiles or refrain from using the term altogether since it is no longer a meaningful form of classification.

This is a simplified cladogram, showing the relationship of birds (aves) with dinosaurs.
http://palaeo.gly.bris.ac.uk/melanosomes//cimages/multimages/16/pic-phylogeny.jpg

Systems in Phylogenetics

Latin names are very common in science, and phylogeny in particular, because Latin is a stable language that is not often used for everyday speech. Since Latin is no longer a living language, it does not continue to develop slang or otherwise change in the meaning of its words. That's why it's used for scientific and legal matters; often these concepts have been stable for hundreds of years. Another benefit of using Latin for such pursuits is that everyone in the profession can understand what is being addressed. When a scientist talks about a bufo vulgaris, for instance, other scientists know that the common toad is the subject of the discussion, regardless of their first language.

Organisms are arranged into groups called clades, which are classified according to shared ancestry. A diagram of clades and how they are differentiated among one another is called a cladogram (see above). The usefulness of a cladogram is showing shared ancestry. Organisms with fewer branches between them are more closely related and share a more recent common ancestor. Organisms that are extremely close together may hybridize to produce offspring because of their genetic similarity.

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