Feel free to discuss this and other assignments with other students in the class. You can learn as much from them as you can from the TA and myself (sometimes). Reconstructing phylogenetic relationships and assigning species to higher taxa on the basis of those relationships is one of the primary goals of many paleobiologists. This exercise is designed to give you practice in the use of taxonomy and phylogenetic systematics ("cladistics").

4.1 Introduction to Systematics

If it were not for taxonomy and sytematics, our understanding of the relationships among living things would be formless and chaotic. Taxonomic classification provides the hangers on which we organize our knowledge of morphology and express hypotheses of relationships. The classification system that we use today was established by Carolus Linnaeus, a Swedish biologist, in the 18th century. It provides a hierarchy of categories used to classify each living organism. From the largest (most inclusive) to the smallest (least inclusive), these categories include:

Order of Hierarchy	Mnemonic (memorization device)
Kingdom	Kings
Phylum	Play
Class	Chess
Order	On
Family	Funny
Genus	Green
species	squares

In our studies of the actual organisms, we will focus mainly on the higher categories of phylum and class. We will deal with lower levels, such as specific genera and species principally when a specific one helps illustrate some important concepts (otherwise the names just get too numerous!). In order to keep information organized however, you will need to remember the order of this hierarchy. One way to memorize the order of these categories is to use a mnemonic such as the one above, or, if you prefer, make one up yourself.

4.1.1 Species Concepts and Binomial Nomenclature

Linneaus established in his taxonomy a system of binomial nomeclature, in which every living organism has a binomial ("two-part name") consisting of its genus name and species name, in that order. The best-known example of a binomial is Homo sapiens, the genus and species to which modern humans belong. The first letter of a genus name is always capitalized; the species name is never capitalized, and never appears without the generic name of its initial. Both names must have a certain Latin form and, for this reason, are underlined or italicized. All countries use both binomial nomenclature and the hierarchy of categories established by Linneaus (sub-categories are also often added); this makes it possible for systematists around the world to use a common system of names, regardless of their native language. If this seems odd, stop and imagine, for just a moment, what would happen if we all had to memorize a taxonomic name in all its possible translated forms. Can you imagine the confusion and extra names that might result? (It sometimes happens anyways)

For students, learning classification is one of the first steps toward sorting out the many new names and morphological characters that appear each lesson. At least at first, students usually prefer that classification remains as unchanging as possible. After all, who wants to see five alternate classifications, when one is coomplicated enough? Systematists, however, are part of a dynamic science for which an unchanging classification would represent a lack of progress in understanding the organisms they study. Both new evidence, such as new fossil discoveries, and new approaches constantly change our way of looking at relationships.

One of the most important changes in systematic thinking in recent years has been a shift to the use of phylogenetic systematics or "cladistics", as some people call it. According to phylogenetic systematics, whose philosophical father was a German entomologist named Willi Hennig, classification should always reflect evolutionary relationships of the organisms involved. Hennig's recommendation may sound obvious, but in fact most classifications were not-and many still are not-constructed strictly on the basis of evolutionary relationship.

4.1.2 Taxonomy vs. Systematics

Systematics is not taxonomy (and vice versa). Taxonomy is merely the classification and naming of organisms. It is not necessarily dependent on systematics, but most people agree that it should be. Grouping organisms according to the extent to which they are related is the most logical method, but other (subjective and somewhat less logical) methods have been used however. Taxonomy is useful for identifying and discussing organisms. Systematics is a scientific discipline in its own right.

4.1.3 Phylogenetic Systematics

Phylogenetics is a method for determining the relationships of organisms. The goal is to try to understand which organisms are most closely related to eachother. This is done by comparing ancestors and descendants.

4.1.3a How does it work? Trees . . .

Evolutionary relationships are represented by phylogenetic trees. In this section you will get familiar with the terminology used to describe trees and the relationships of the organisms on those trees.

Phylogenetics is a method for determining the relationships of organisms. The goal is to try to understand which organisms are most closely related to eachother. This is done by comparing ancestors and descendants.

You may be wondering how this is different from taxonomy. Taxonomy is merely the naming and classification of organisms (Remember: Kingdom, Phylum, Class, Order, Genus, Species). Phylogenetic systematics however, attempts to go beyond just that. Phylogenies provide a hypothesis for the evolution of organisms

Figure 4.1 - Some tree terminology.

Figure 4.2 - Groups on a phylogenetic tree.

A monophyletic group is often called a clade. It includes an ancestor and all of its descendants.

A polyphyletic group is one that is derived from many ancestors.

A paraphyletic group is one that includes the ancestor, but not all of its descendants.

Sister groups share a common ancestor and are each other's closest relatives.