|Plant Taxonomy (BIOL308) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; email@example.com; http://www.employees.csbsju.edu/ssaupe/|
I. Cladistics - "new" kid on the block
A. Willi Hennig (1913-1976)
Zoologist (entomologist, German) - developed this technique in 1950. The original publication was in 1950 but it was not translated into English until 1966.
B. Clade - lineage (klados, Greek = branch)
D. Produces a branching diagram - cladogram
Examine a cladogram (in your text or provided in class). Note: (a) the nodes of the cladogram represent speciation events where ancestral species split into descendants; (b) internodes - region between nodes; (c) sister taxa - derived from the same node; (d) cladograms can be rooted or unrooted; (e) the taxa are at the ends of the branches, there are no intermediates at the nodes; and (f) a cladogram (or phenogram) can be rewritten as a Venn diagram (clustered circles)
II. Cladistic vs. Evolutionary Classification
A. Both methods assume that:
B. Comparison Table
|Focus||degree of advancement and divergence from a common ancestor||branching pattern of evolution|
doesn't generate hypotheses, classification generated by personal experiences with material
|generates testable hypotheses about branches and tests by collecting data|
III. Cladistics - Specifics
Cladograms are based on shared derived characters. A derived or advanced character is called an apomorphy. A primitive character is called a plesiomorphy. If a character is derived, it is assumed to be a "step up" from the primitive, plesiomorphic condition. Plesiomorphies arose before the taxon evolved, whereas apomorphies evolved with the taxon. Advanced characters shared by taxa are called synapomorphies, while shared primitive characteristics are symplesiomorphies.
B. Homologous characters
These are features shared by common ancestry, equivalent in an evolutionary sense. For example, various types of mammal forelimbs are homologous characters. In contrast, bird wings vs. insect wings are analogous characters and not equivalent. A cladogram (or evolutionary classification, for that matter) based on analogous traits clearly would be flawed. A cladogram must be based on comparing homologous characters. This is easier said than done, especially considering the problem of convergent evolution. For example, if we used succulence to develop a cladogram, we might erroneously group certain members of the Euphorbiaceae with the Cactaceae. And, consider underground structures of plants. Rhizomes, corms, tubers, and runners are all stems and therefore, homologous structures. However, the potato and a sweet potato are not homologous since one is a stem (=potato) and the other a root (sweet potato).
Assume that you have three taxa. How many possible cladistic relationships exist between them? Four (see your text or in class). As the number of taxa increases, so does the number of possible cladograms [for example with 4 taxa there are 26 possible cladograms, with 5 taxa there are 125, and with 7 taxa there are a whopping 7 trillion possible cladograms!]. So, how do you determine which one is "best"? In practice, this is somewhat complex, but in theory it's relatively simple. Let's use some "fun" examples:
IV. Chain Letters & Telephone - Models for Cladistics
A. The Game of Telephone
Did you ever play "telephone?" Recall that in this game that there is a chain of people and that the person at the beginning of the chain whispers a message to next individual who secretly whispers it to the next and so on all the way down the line. By the end of the line the message is usually garbled in a humorous way. This game provides a model for how cladistics works by imagining that the path of people is not straight but branched, much like the branching pattern by which taxa evolve. Thus, in our new game we would have branches where individuals at the nodes would relay the same message to two different individuals and so on. Can we reconstruct the pathway of individuals if we just know what the end message is? You bet - and that's exactly what cladistics does. Taxa are like the message at the end of the chain and we can reconstruct the branches by comparing the taxa to the presumed original message (or likely ancestor of the group). Click here for an example/exercise of "the cladistic telephone."
B. Chain Letters
Chain letters are another good model for cladistics. Over time, chain letters change or "evolve" and cladistic methodology can be used to track the most likely changes. In an article in Scientific American, Bennett et al (June 2003) argue that changes in these chain letters can be analyzed much like changes in the nucleotide sequence of DNA. Using a series of 33 versions of a chain letter they had collected, they were able to demonstrate the probably path of changes. Check out the article.
C. Conclusions: Note that in both cases, the original "message" changed (mutated) as it was passed from individual to individual. These changes are analgous to the apomorphies, advanced derived features, that evolve in various taxa. Just like the changes in chain letter and game of telephone can be used to reconstruct the history of message passage, so too can the apomorphies be used to reconstruct phylogenetic history.
V. How to do a Cladistic Analysis
A. Cladogram Construction.
Click here for an exercise creating/analyzing a cladogram.
VI. Disadvantages of
The final cladogram may include examples of homoplasy - a situation where unexpected characters occur in a taxon. These may result from convergent evolution (unrelated taxa evolve similar features because of similar selection pressures) and parallel evolution (taxa that diverged from a common ancestor evolve in a similar manner). Problems can arise from the wrong outgroup being selected or reversal of characters to the plesiomorphic state. Other problems include:
VII. Advantages of cladistics
Reproducibility and objectivity!
08/20/2007 / � Copyright by SG