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.
CPSP118G Spring Semester: Earth, Life & Time Colloquium
Representing and Analyzing the Pattern of Evolution
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.
- By interbreeding I don't mean that everyone mates with everyone else, but that over the long term there is no barrier to gene flow throughout the population.
- Organisms that don't contribute genes to a lineage don't qualify as members, either because simply never interbreed (E.g. Humans and gorillas), or interbreed but don't produce viable/fertile/fit offspring. (E.g. horses and donkeys, which produce sterile mules).
- As populations interbreed, the various processes of evolution are acting on them, allowing some novel characters (new heritable features) to spread through the population and extinguishing others.
Cladograms: Graphically describing patterns caused by speciation.
- If we placed a physical barrier between members of our lineage here, then:
- the individuals on one side of the barrier would be deprived of the genetic material on the other side, thus any modifications that occurred in one group could not be passed on to the others
- The populations on different sides of the barrier would begin to accumulate different sets of modifications.
- Ultimate, they might diverge morphologically to the point that
if they ever reestablished contact, The two groups would be sufficiently dissimilar that they could not or would not mate and produce evolutionarily fit offspring. At this point, it would be very unlikely that the two lineages would ever merge. Lineages that split in this manner have undergone Speciation.
- Speciations occur because:
- Members of a population are geographically separated
Example: Canada geese and Nene or Hawaiian geese.
- Members of a population become ecologically specialized and have no chance to mate. E.g.: Acheta: In the east US there are two species of gray cricket that are thought to derive from a recent common ancestor. In both, juveniles are extremely vulnerable to cold. Their responses to this problem differ. One species mates in the fall, eggs are laid which overwinter and adults die. The other overwinters as adults and mates in the spring. by next fall, the juveniles are grown and overwinter as adults. Common ancestor lived in a warmer climate and mated year round.
- An evolutionary novelty prevents some members of a population from mating with others Busycon: Sudden appearance of sinistrality in marine snails prevents mating.
- Graphic representation of a speciation. I have been drawing an elaborate picture of a lineage. From now on I will abbreviate with a line. Where a speciation occurrs, I will draw a Y shaped bifurcation.
- Suppose that in the course of history, a lineage splits many times? The result is a complex evolutionary tree. We can describe this graphically in the form of a cladogram, a stick figure tree that breaks down into several speciations.
The branching that you see here is the essence of the pattern of evolution. Every time a lineage splits, an identifiable group of lineages is born.
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.
- A hierarchy of internested groups, with those descended from more recent common ancestors being nested within those descended from more distant ones. For instance, "Tetrapoda", the common ancestor of land vertebrates and its descendants, is nested within "Choanata", the common ancestor of vertebrates with choanae and all of its descendants.
- An organizing principle, the branching pattern of evolution itself.
Evolution as Pattern II
- Phylogeny: The branching evolutionary pattern of ancestry and descent.
- Monophyletic groups: In phylogenetic systematics, taxonomic groups are defined strictly in terms of the non-arbitrary criterion of descent from a common ancestor. Such taxa are called monophyletic groups.
Memorize this definition: A monophyletic group is an ancestor and ALL of its descendants.
- Synapomorphy: A shared derived character. I.e., an evolutionary novelty shared by more than one taxon on a cladogram. E.G.: In the vertebrate cladogram above, the presence of fingers and toes is a synapomorphy of Tetrapoda. They're ancestrally present in amphibians and land vertebrates but absent in lampreys, sharks, ray-finned fish, and lungfish.
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.
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
- 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.
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.
- One has long antennae while the others have short ones.
- One has plain wing covers while the others have spotted ones.
- One has stripes
- Two have stalked eyes while the third does not.
- 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.
- 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:
- Most reasonable hypothesis of phylogeny based on available data, not necessarily truth.
- Model of character evolution: Note homoplastic characters. We can identify likely points of character state change, and distinguish convergent characters from unidirectionally evolving ones.
: 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: