CPSP118G Spring Semester: Earth, Life & Time Colloquium

Representing and Analyzing the Pattern of Evolution

The goal of systematics: The diversity of living things presents us with a seemingly infinite variety. The science of systematics is dedicted to identifying and ordering the diversity of living things.

How do we "order the diversity of living things?" - by referring to the pattern of the tree of evolution. First, a little review:

Where does the diversity come from and why does it form a tree?

Lineages: Interbreeding group of sexually reproducing organisms projected through time.

II. Speciation.

Cladograms: Graphically describing patterns caused by speciation.

In this cladogram, the organisms A, B, and C at the ends of the branches are known as terminal taxa. The lines themselves represent evolving lineages. Branch points represent lineage splitting events. The point at the fork of each split is called a node, and represents the latest common ancestor of the descendants depicted above it. Time runs from oldest events at the bottom to youngest ones at the top. Thus, in this example, the last common ancestor of A, B, and C occurred earlier in time than the last common ancestor of B anc C.

Note that in a cladogram, it does not matter whether things apear on the left or right. What counts is the sequence of branching events (i.e. which ones appear on top or on the bottom). In the figure above, cladograms 1 and 2 depict exactly the same relationships, whereas cladogram 3 is different.

Why we care: The phylogenetic taxonomic system: A cladogram represents the flesh and blood tree of evolution. Remember, that is the pattern by which systematist organize diversity. Taxonomic groups can be named and defined based on their descent from a common ancestor. The cladogram below shows the real relationships between several major vertebrate groups.

Working from this cladogram, systematists have named the following taxonomic groups:

In this drawing, we have drawn circles around the groups that could be defined by the relationships shown on this cladogram, and indicated their names. Ordinarily, one would simply write the group names next to the node of the last common ancestor:

BUT NOTE: There are many vertebrates that we are not showing on this cladogram. (chimaeras, coelacanths, and scads of extinct critters.) Furthermore, each of the terminal taxa could be expanded into its own large cladogram. A given cladogram can only represent the evolutionary pattern of a subset of the overall tree of life.

The pattern of evolution provides:

Presto! It's a proper taxonomic system.


Evolution as Pattern II

The Phylogenetic System of Taxonomy: The organizing principle of modern biology is evolution (descent with modification). Ultimately, evolution implies that all living things descend from single common ancestor. The history of these lineages is their phylogeny. (We already know how to draw it). This supplies the organizational principle used by modern systematists. It is hierarchical because groups that are descended from very recent common ancestors may be nested within groups descended from distant common ancestors.

If God were to hand us the true phylogeny, and our only task were to read it and construct taxonomic system accordingly, our lives would be easy. Instead, we must somehow reconstruct phylogeny by making observations and testing hypotheses. This is where the "modification" side of "descent with modification" comes in. As lineages evolve, the characters of their members change. I.e. they go from ancestral to derived states.

  1. Sometimes we refer to cladograms as "trees." It helps to think of them that way. Imagine marker dye injected into branch. In the tree of evolution, the derived character states play the role of the marker dye.

  2. There is a particular type of derived character state that we are particularly interested in. Synapomorphy = Derived character state shared by different lineages.
Synapomorphies allow us to identify monophyletic groups, because if a character is shared by two lineages, we assume that it was inherited from their most recent common ancestor

Let's see how this works in a simple cladistic analysis of some imaginary beetles. We assume that they are related somehow, but we don't know if B shares a more recent common ancestor with C or A, or if C and D are more closely related to one another than to B.

  1. The first thing we do is notice how they are different.
    1. One has long antennae while the others have short ones.
    2. One has plain wing covers while the others have spotted ones.
    3. One has stripes
    4. Two have stalked eyes while the third does not.

  2. We compile this information in a table called a taxon-character matrix.

    1. Large jaws present
    2. Small antennae present
    3. Spots present
    4. Stripes present

    This matrix records whether the observed state for each taxon is ancestral or derived. How do we know? You may have noticed that we we haven't had much to say about A. In this analysis, A is the outgroup taxon. This is a beetle that, on the basis of some prior information, we can assume is more distantly related to beetles A, B, and C than any of them are to one another. Maybe we found it fossilized in amber. The outgroup is our standard for what is derived and what isn't, in that anything we see in it, we assume to be the ancestral state. Incidentally, because it has smaller mandibles than the others, I've included a "large mandible" characteristic in the matrix.

    Tree 1:

    Tree 2:

    Tree 3:

  3. We now compare every possible evolutionary tree arrangement by mapping onto them the simplest possible distribution of each character state change.

    principle of parsimony, which holds that the simplest solution is usually the best. For us, the simplest hypothesis is the one that invokes the fewest character state changes. That is tree 1, with only six changes. Is this the true tree? God only knows. The best we can say is that it is our best approximation based on current knowledge.

IV. What does this method yield:

  1. Most reasonable hypothesis of phylogeny based on available data, not necessarily truth.

  2. Model of character evolution: Note homoplastic characters. We can identify likely points of character state change, and distinguish convergent characters from unidirectionally evolving ones.

Potatohead Exercise: The take home concept is that the hypothesis of phylogeny that our technique generates is falsifiable. We can falsify it by adding new information or changing basic assumptions like outgroup choice.

Feeling vulnerable? For more review see: