Plant Taxonomy (BIOL308) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/ |
Cladistics
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)
C. Theory
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
Evolutionary Systematics | Cladistics | |
Focus | degree of advancement and divergence from a common ancestor | branching pattern of evolution |
Approach | intuitive | analytical |
Scientific Approach |
doesn't generate hypotheses, classification generated by personal experiences with material |
generates testable hypotheses about branches and tests by collecting data |
III. Cladistics - Specifics
A. Characters
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).
C. Cladogram
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.
B. Exercise
Click here for an
exercise creating/analyzing a cladogram.
VI. Disadvantages of
Cladistics
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!
References:
Software Packages:
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Last updated:
08/20/2007 / � Copyright by SG
Saupe