Biologists need a sensible system for organizing the complex array of living things.

A systematic approach results in clear and testable hypotheses about the relationships between organisms. Biologists make judgments about how organisms are related based on observations of similarities between them. Unfortunately, similar characteristics can result from adaptations to a similar environment, even in unrelated groups of organisms.

For example, while a dolphin may look more like a shark or a tuna, it’s actually more closely related to a cow! The similarity of dolphins to fish is the result of convergence, the independent evolution of similar characteristics in unrelated taxonomic groups. Luckily, careful analysis can reveal relationships even when convergence is a factor.

In systematics, a hypothesis usually takes the form of a phylogenetic tree. This type of diagram shows the evolutionary relationships of organisms, with known species or groups of species at the leaves, and common ancestors at the base and at the forks of its branches.

At each fork, a common ancestor gives rise to two descendants. This splitting of lines of descent can continue indefinitely to the final "leaves" of the tree.

No one can claim to know details about the common ancestors in a phylogenetic tree. The goal of the tree is to show the relationships among species we do know about, including extinct species for which there may only be fossil evidence.

Here's an example of a phylogenetic tree that shows how some mammal species are related. Notice that each type of mammal is at a leaf of the tree. Each fork in the tree represents the point where a line of descent diverged to give rise to two species or two groups of species. Any group of organisms which have all descended from a particular common ancestor is called a clade, from the Greek word for "branch". When a group of organisms is given a name by taxonomists, it becomes a taxon.

Most taxonomists today prefer to create taxa from single clades; when they do, the result is a monophyletic taxon, a "one-tribe arrangement." One clade in this tree that’s also a monophyletic taxon includes whales and dolphins and their relatives. It name is Cetacea.

The vertical distance in a phylogenetic tree is a measure of the evolutionary separation of a species from its ancestor. This may be the time back to the prehistoric ancestor, or it may be derived from another measure of separation, such as the differences in DNA or protein sequences.

A diagram that's simpler than a phylogenetic tree, showing only the branching of the lines of descent, is called a cladogram. A cladogram doesn’t give a measure of the evolutionary separation between organisms at the leaves of the tree. Constructing a cladogram for a set of species or taxonomic groups is one of the first steps in creating a phylogenetic tree.

Let's consider how a simple cladogram might be constructed. We’ll need a set of organisms or clades to analyze. We'll use the following set for our example: the alligator, the tuna, the leopard, the chimpanzee, and the shark. We also need a related species or clade to use as a standard for comparison. This is called an outgroup. We must be sure that the outgroup branched off the main line of descent of the set we're studying before the last common ancestor of that set. For this example, we'll use a lamprey, a type of jawless fish.

How do we go about comparing the organisms? We need a list of traits that distinguishes the members of the set. A trait present in the outgroup, and therefore probably present in the common ancestor of the set of organisms being studied, is an ancestral trait. We're interested in using traits that show differences from the ancestor because of modifications of ancestral traits. These are called derived traits. We wish to group organisms with a shared derived trait, assuming that it arose in an ancestor common to that group.

Proper selection of traits is one of the most challenging aspects of this type of analysis. For simplicity, we'll use a small set of traits, which are either present or absent in a given organism. We're using only very obvious traits – traits describing the form of an organism's body or its body parts. The physical form and structure of the body of an organism, and the study of this, is called morphology. So the traits we’re talking about are morphological traits.

This table lists the traits for our example. Biologists typically use a large number of traits, including morphological, molecular, behavioral, and biochemical traits. Also, traits may vary gradually, rather than simply being present or absent. Computers are often used to handle all the data.

For our example, let's note in the table which organisms possess each trait. Each branch point in the cladogram will be associated with a particular derived trait. Our goal is to arrange the organisms so that they are distinguished by the trait associated with the branch point. The jaw is a trait of all the species except the outgroup, the lamprey, which is a jawless fish with a sucker mouth. All the other organisms have the jaw as a shared derived trait. The branch point at the base of the cladogram is associated with the jaw.

Even though the structure of the jaw has been modified by evolution, it’s a recognizable feature distinguishing organisms that possess it. A structure such as this, inherited from a single common ancestor and usually modified by evolution, is called a homologous structure. By contrast, analogous structures have similar functions and appearance but are derived from unrelated ancestral structures via convergence.

Now for the next trait in the table.

Of the organisms on the right, only the shark lacks a bony skeleton. The next branch point, separating the shark from the alligator, tuna, leopard, and chimpanzee, is associated with a bony skeleton. The next traits are lungs and 4 walking limbs. The tuna is the only organism of the group on the right that lacks both these traits. Let’s split off the tuna with the next branch point. The alligator lacks fur or hair, so it's the next to branch off to the left.

The lack of a feature like a tail can be taken as a derived trait, and in our example, only the chimpanzee possesses this trait. It remains at the far right of the cladogram, separated from the leopard by the final branch point.

Now it's your turn. Try placing the four organisms into their correct locations in the cladogram. Use the same traits as we used in the example, and keep the lamprey as the outgroup. Click "Submit" to check your answer. Click "Jump Ahead" to skip this step.

Sorry, that's not correct. The organism at the far right must have a trait distinguishing it from all the other organisms.

Sorry, that's not the best solution. The organism closest to the lamprey should have the fewest traits different from this outgroup.

Sorry, that's not correct. Do you want to try again?

That's correct! The goldfish lacks lungs and four walking limbs, separating it from the lizard, dog, and human. The lizard's lack of hair or fur distinguishes it from the clade containing the dog and the human. And the lack of a tail is the trait distinguishing the human from all the other organisms.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education