Phylogeny:
the reconstruction of evolutionary histories of animals.

Spend some time studying this multi-page website about the application of the scientific method to understand the evolution of life on Earth.

An understanding of evolution and classification must come before an understanding of phylogeny. You know that "evolution" is a process by which organisms change over a period of time. You also know that there is no "right way" to classify organisms. But there is a "natural way". Organisms are a product of their genetic history. There are mutations, from time to time, that cause slight changes in the gene pool of a group of organisms. As long as the organisms freely breed within the gene pool, there will be slight variations, but nothing more.

If something happens that separates a group from the original gene pool, some of the original variations are lost and new ones arise. Over a period of time, this can cause the separated group to have characteristics that are not found in the original group. How far one follows this road is up to the individual, but do not lose sight of the definition of a "species". A species is a group of organisms that can mate and produce fertile offspring. Phylogeny should not be though of as a way of identifying speciation, the development of a new species from an old one.

Constructing a phylogenetic tree:

1. Any reconstruction must be based on some hypothesis, such as:
   1. Animals having similar developmental stages have a similar evolution.
   2. An individual's development repeats the phylogeny of the group the which the individual belongs.
   3. Animals with structural similarities have an evolutionary relationship.

2. Using this hypothesis as a starting point, it is possible to construct the phylogenetic tree, a hypothetical model of animal evolution. These models do not imply that any living group is the ancestor to any other group of animals. They simply represent interpretations of animal history based on evidence of relationships.

3. There are three types of phylogenetic groups of animals
   • Monophyletic - A group including a given ancestral form and all of its recognized descendant species.
   • Paraphyletic - A group including a given ancestral form and some of its recognized descendant species.
   • Polyphyletic - A group including the descendants of more than one ancestral form.

4. While there is no "standard" for drawing phylogenetic trees, most provide information using:
   • Branch Order - The branching pattern shows the genealogy of the organisms. The closer two organisms are to a branch, the closer their relationship. One might think of a branch as the place where a gene pool is separated.
   • Branch Length - The length of a line on the tree represents an amount of change. The longer the line between two organisms, the greater the difference between them. The length of the line does not indicate a length of time, only the degree of change.

In this example, A and B share the most recent common ancestor, C. Because of this, A and B are the most closely related. Organism C is the common ancestor of the group composed of A and B, which makes C equally related to A and B. The oldest organism in this tree is D, which is the ancestor of the group composed of A, B and C.

The importance of the phylogenetic tree model does not lie in its representation of actual events. The model is to stimulate thinking and discussion about how the major groups of animals have evolved and about which sources of data provide the best background. A phylogenetic tree also provides an organized outline from which students can begin their study of animal diversity.

Research Links:

• Strange Science - Michon Scott