Introduction

Biology is largely a comparative science. This should be easy to understand, considering the nature of variation in the natural world (see exercise 1). We see organisms as composites of characters, and character states, which are identifiable expressions of the genetic makeup of organisms. If you were to describe a giraffe, you might consider its long neck, its brown and yellow coat, and its long, black tongue. Thus, you have described a number of characters - neck length, coat color and pattern, and tongue size and color. It is important to recognize characters and character states when describing organisms, and as you’ll see below, it is even more important to think in these terms when classifying organisms.

(A) Classification

Humans have always found it useful to recognize, name and classify things, because this is a prerequisite for communication. We classify rocks, mountains, seas, types of snow, and, in fact, everything with which we come in contact. Classification and nomenclature of plants and animals has been practiced by nearly every human culture throughout history. For example, Polynesian peoples have given names to nearly all of the diverse types of fish inhabiting coral reefs; Australian Aborigines have names for thousands of plants and animals; it is likely that early humans had classifications enabling them to distinguish the many kinds of organisms with which they interacted.

Classification involves the sorting of three or more entities. It is impossible to classify, in the systematic sense, one entity - all you can do is describe it, but you can’t make any statement about relationships. It is also difficult to classify two entities - you can differentiate them, but again, you cannot say anything about their relationship with each other. With three entities, however, you can say that two are more closely related to each other than either is to the third entity. We will review the use of the “three-taxon statement” later in this exercise.

(1) Taxonomy

The branches of biology that are concerned with classification are taxonomy and systematics. Taxonomy is the science of classification, including the description, naming, and assignment of organisms to more inclusive groups.

(B) Systematics - the study of Phylogeny

Systematics is the study of relationships among organisms - who’s related to whom, and processes by which groups become differentiated from a common ancestor. Some biologists do not differentiate the terms "taxonomy" and "systematics", appreciating that the two are closely interrelated. A major goal of systematics and taxonomy is to answer the three-taxon statement - given three taxa, which two are more closely related to each other than either is to the third?

While there may be many different goals in constructing a taxonomic treatment, most evolutionary biologists are interested, among other things, in deciphering the evolutionary history of species - i.e., the phylogeny of a group. Of course, it is impossible to travel back in time to witness the evolution of species from their ancestors. Therefore, systematists seek to reconstruct phylogenies via an array of approaches.

(1) Approaches to Reconstructing Phylogenies

Some systematists (often known as Evolutionary or Traditional Systematists) do not employ an explicit method, but rely on enormous amounts of information and the researcher’s personal experience with the group in question. Another approach, Phenetic Systematics, assesses relationships based on overall similarity of members of a group. Phylogenetic Systematics (also called cladistics) uses only shared derived characters to assess relationships. Phlylogenetic systematics is the main method used in systematics today.