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.
- 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.
- There is a particular type of derived character state that we are particularly interested in. Synapomorphy = Derived character state shared by different lineages.
Note: Just as we have disphonious cladobabble describing different types of taxonomic groups, we have it for characters, too:
- Plesiomorphy: The ancestral character state - inherited from distant ancestors. E.G. From the point of view of Primates, having five fingers is a plesiomorphy. (Symplesiomorphies are "shared ancestral states.")
- Apomorphies: A derived (evolved) character state. E.G. From the point of view of Primates, having an opposable thumb is an apomorphy.
- Synapomorphies: are "shared derived states." E.G. the presence of an opposable thumb is a synapomorphy of humans, chimpanzees, monkeys, lemurs, and other lineages of primate.
- Autapomorphies: Unique derived character state. E.G. The enlarged braincase of humans is an autapomorphy, not shared with other animals.
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.
- The first thing we do is notice how they are different.
- Three have big jaws while one does not.
- Two have long antennae while the others have short ones.
- Two have plain wing covers while the others have spotted ones.
- One has stripes
- 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
Outgroup taxon: 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 taxon 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 B, C, and D 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.
- We now compare every possible evolutionary tree arrangement by mapping onto them the simplest possible distribution of each character state change. Because A is presumed to be the outgroup, its ancestral lineage always branches off first, no matter what.
The evolutionary novelty of large mandibles, for example, can be explained most simply assuming that it appeared once before the last common ancestor of A, B. and C, who simply inherited it. Short antennae are a different story. The outgroup has long antennae, so we assume this is the ancestral state, but beetle C retains them also. In some tree arrangements, this implies that short antennae they evolved independently in A and B. After mapping the character changes onto the trees, we count them up. Tree 1 implies a minimum of six changes, tree 2 suggests a minimum of eight and tree 3 would require a minimum of seven. Each of these arrangements is a plausible hypothesis of evolutionary history. Now, we must choose the best hypothesis. To do that, we use the 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.
- Most reasonable hypothesis of phylogeny based on available data, not necessarily truth.
- Model of character evolution: We can identify likely points of character state change, and distinguish convergent characters from unidirectionally evolving ones. Note: Not all characters evolve concordantly. Some experience reversals or convergences. The phylogenetic analysis helps us identify these homoplastic characters.
Feeling vulnerable? For more review see: