Phylogeny Part 2: Introducing Cladograms (Click Here
to Read Instructions)
THANK YOU FOR SUBMITTING YOUR ANSWERS TO PART
1!
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reference.
The cladograms you learned to interpret in Part 1 of this exercise
might have seemed difficult to conceptualize because they were merely branches
and names. But cladograms are highly useful guides to a group of organisms.
A particular cladogram hypothesis immediately suggests the genealogical branching
pattern. For example, consider this cladogram from Part 1:
For review, the reason I said "cladogram" and not
"cladograms" is that these are the same cladogram, just drawn a bit
differently. In this cladogram, for example, a bird and crocodile are sister
taxa, relative to a snake or lizard, or even more distantly related vertebrates.
That information about branching pattern is useful in itself, but branching
pattern alone does not really tell us much about the group or the sorts of evolutionary
changes that occurred in it. Plus, one has to assume that there is some good
reason why this cladogram is drawn, and not some other alternative cladogram,
for example, one that puts birds and mammals together. After all, both birds
and mammals are endothermic (i.e., "warm blooded"), and crocodiles,
lizards, and snakes are not.
So Part 2 and later parts will be about how evidence is accumulated
in support of one or more cladograms, to the exclusion of others. It is about
the parsimony criterion, which is used to choose between alternative cladograms.
Moreover, it is about how to interpret the evolution of characters (i.e., traits
of organisms), which connects cladograms to notions like adaptation.
Consider again the cladogram above. It would
be natural to be interested in the evolutionary history of land vertebrates,
and the sorts of evolutionary innovations that might have occurred that might
have been important in explaining patterns of diversity through time. We might
especially be interested in the sorts of adaptations (or preadaptations = exaptations)
that allowed some groups to become fantastically successful. In old-fashioned
treatments of evolution, the movement of a group into a new "adaptive zone"
(e.g., evolving powered flight as in birds) was often given great emphasis in
tree diagrams (for example, in
the writing of G.G. Simpson), but when these notions were added to tree
figures the actual pattern of evolutionary branching often became obscured.
Click here
to open up a new window with an example of one of these sorts of "bubble
diagram" trees.
CQ1 (1 pt): Look at this "bubble diagram" tree and use it to formulate a
short conjecture (about 25 words or less) about what sort of ancestor a bird
might have evolved from. Please do this before you proceed further. If you
are really a glutton for punishment, attempt to draw a cladogram version of
this figure.
Note, this "bubble" figure is from the current edition of the leading
college zoology textbook, which should tell you something about the gap
between what we now know and what current textbooks are presenting.
In contrast, cladograms are based strictly on the hypothesized pattern of
splitting events. This approach has the advantage of allowing an
investigator to immediately see the hypothesized sister taxon relationships
between taxa, but there is another important advantage. A cladogram also
allows the investigator to present important events in the evolutionary
history of the group.
One clarification is necessary before an example is presented. Some events
might be important because they were "innovations" that led to explosive
radiations. Other events might be important only because they allow someone
to recognize living or fossil organisms as members of a group, and might
actually have very little, if any, "adaptive" value.
Now we are ready to look at cladograms that correspond approximately to
the bubble diagram above. Look at several cladograms from
Sandy
Carlson.
CQ2 (1 pt.): Now, repeat what I asked you to do above in CQ1. Hypothesize
what sort of ancestor gave rise to birds, only this time use the cladogram
views. If you forget what a clade is, remember the
"snip rule."
CQ3 (2 pts.): Briefly contrast the descriptions derived from the "bubble
diagram" tree (CQ1) and the cladogram based on characteristics mapped on
seven living taxa (CQ2). What can you conclude about these alternative
approaches with respect to making statements about the evolution of traits?
Note that a cladogram is a good example of an "unranked classification." The
"unranked" means that none of the taxon names are preceded by a rank (e.g.,
phylum, class, order, family, etc.). Because cladistic classifications
strictly conform to the "rule of monophyly" we do not need to worry about
ANY of these taxon names being paraphyletic. That is, each taxon includes
all of the taxa indented directly underneath it. For example:
The taxon Theropods includes everything indented underneath
it, including the daughter taxa, Ceratosaurs and Coelurosaurs plus everything indented underneath
Coelurosaurs, including its daughter taxa, Ornithomimosaurs, Dromaeosaurids, and birds. Likewise, characters are
hierarchically arranged, just like in nature. The taxon Theropods is
diagnosed as having sharp claws on grasping hands. This is not a
"definition" of Theropods, it is a diagnosis. The diagnosis is what is
inferred to have been in the common ancestor of Theropods, based on a
technique in phylogenetic analysis known as "character optimization." The
important point here is that if one of the descendants become secondarily
modified to lose their sharp claws, it does not mean that this descendant is
then somehow kicked out of the group. Think about a penguin. Does it have
sharp claws? Do we have trouble thinking about it as a bird? Why should we
have trouble thinking about it as a Theropod?
Perhaps the biggest advantage of an indented classification like this is
that it exactly corresponds to the cladogram hypothesis. You can construct a
classification directly and unambiguously, given a cladogram. It works in
reverse as well. Given an indented classification, you can turn it into a
cladogram. If you would like to know how to turn an unranked indented
classification into the corresponding cladogram, follow these instructions:
http://biology.fullerton.edu/courses/biol_404/web/hierarchies.html
From a student's perspective, having a classification that corresponds
exactly to the best cladogram available simplifies studying considerably,
because once a cladogram is learned, the classification naturally follows.
That is not true of conventional classification systems, because many
conventional taxa are paraphyletic. For example, Reptilia and Aves are at
the same rank in a conventional classification, even if everyone agrees that
Aves is nested within reptiles on the best supported cladogram. In other
words, no one is disputing the cladogram, only how to best construct a
classification system. The cladistic approach is simpler, albeit requiring
you to give up familiar paraphyletic taxa.
Take a minute and look at this
dinosaur
classification and note how Ceratosaurs and Coelurosaurs are displayed as daughter taxa of Theropods.
CQ4 (2 pts.): Extend this reasoning to complete the following sentence. The
taxon _____________ is the sister taxon of Theropods, and these are daughter
taxa nested within the higher taxon ____________. One trait that unites
members of this higher taxon is ___________________.
Once you get the hang of thinking about cladograms, you can look at a new
one, perhaps drawn somewhat differently, with nodes rotated, and with
somewhat different terminal taxa. For example, click here to open up yet
another window of a fun
dinosaur cladogram from the American Museum of
Natural History's OLogy website for kids.
CQ5 (2 pts.): Find the node (blue dot) corresponding to the last shared
common ancestor of birds in the OLogy cladogram, and complete the following
sentence. The sister taxon of birds is _____________________ and within
birds, the living but flightless bird, Eudromia elegans, is sister taxon of
a clade of extinct birds, __________ and Presbyornis pervetus (according to
this cladogram hypothesis).
Extra challenge: If it is related to modern ducks, could Presbyornis
pervetus quack, according its species card on this web page (click on its
name, and then the yellow arrow then you may have to sign in to collect
the card)?
CQ6 (2 pts.): Consult this
Archaeopteryx web page.
Name some derived (new) features that modern birds share that are lacking in
the ancient bird, Archaeopteryx (and also lacking in other dinosaurs). Next,
name some derived features that Archaeopteryx shares in common with those
other dinosaurs known as birds.
Do not expect to find the answer for the last question neatly listed on the
linked web site. Note that in the above question I am emphasizing the
derived similarities only, not the "primitive" (technically termed
"plesiomorphic") similarities that are merely inherited from a more ancient
ancestor. For example, Archaeopteryx had teeth, but so do other reptiles and
mammals and sharks, etc. The fact that both Archaeopteryx and T. rex had
teeth does not help provide evidence that they are closely related. One has
to find derived similarities that were present in their immediate common
ancestor but not in their more ancient shared common ancestor with another
taxon (e.g., the Ornithiscian dinosaur, Triceratops). Note that a derived
similarity can be the loss of a trait (e.g., teeth), provided it is a
derived loss (i.e., was previously present in more ancient ancestors). Here
are some examples of derived character states hypothesized for various taxa
of terrestrial vertebrates. When this assignment is eventually expanded, the
emphasis on shared derived (i.e., "special") similarities hypothesized to be
homologous will be clarified. (Technically, these are termed
"synapomorphies" for "syn" = shared, "apomorphies" = derived similarities.)
Only shared derived similarities, not shared primitive similarities (=
"plesiomorphies"), can provide evidence for the monophyly of taxa. For
example, if one is comparing a turtle, a lizard, and a snake, the presence
of four limbs does not help provide support for a turtle-lizard grouping
because more distant relatives (e.g., mammals, salamanders) also have four
limbs, so this similarity is a shared primitive trait. In contrast, the
presence of two penises in a male lizard and a male snake is a shared
derived trait, because other male reptiles and mammals only have one penis.
Sandy Carlsonšs Figure 7 (CQ2 above) may help you understand this
perspective.
A figure from Sereno (Science 284: 2137-2147, 1999) illustrates major
stages in the evolution of modern avian skeletal design and function:
Many
skeletal innovations of critical functional importance for flight arose for
other purposes among early Theropods, including (1) the hollowing of all
long bones of the skeleton (Theropoda) and removal of pedal digit I from its
role in weight support; (2) evolution of a rotary wrist joint to efficiently
deploy a large grasping manus; (3) expansion of the coracoid and sternum for
increased pectoral musculature and plumulaceous feathers for insulation; (4)
the presence of feathers arranged for display or brooding or both; (5)
shortening of the trunk and increased stiffness of the distal tail for
balance and maneuverability. Archaeopteryx remains a pivotal taxon (6). Key
refinements of powered flight and perching in later birds include the deep
thorax (7) with strut-shaped coracoid and pygostyle; (8) the triosseal canal
for the tendon of the principal wing rotator, alular feathers for control of
airflow at slow speeds, rectriceal fan for maneuverability and braking
during landing, and fully opposable hallux for advanced perching; and the
elastic furcula and deep sternal keel for massive aerobic pectoral
musculature. Ornithothoracine birds diverged early as Enantiornithes
("opposite birds"), which prevailed as the predominant avians during the
Cretaceous, and (9) Euornithes ("true birds"), which underwent an explosive
radiation toward the close of the Cretaceous.
For more detail, you might find the following links to other resources
useful:
EVOLUTION AND SYSTEMATICS by Sandra J. Carlson
http://www.ucmp.berkeley.edu/education/events/carlson2.html
Hypothesis of phylogenetic relationships among the Tetrapoda
http://www.inhs.uiuc.edu/%7Echrisp/lecturefigs.html
American Museum of Natural History
http://www.amnh.org/exhibitions/permanent/fossilhalls/
Classroom Cladogram of Vertebrate/Human Evolution
http://www.indiana.edu/%7Eensiweb/lessons/c.bigcla.html
Introductory Glossary of Cladistic Terms
http://www.science.uts.edu.au/sasb/glossary.html
This page created July 28, 2002, last modified
July 10, 2006.
Copyright © 2002- 2003 D.J.
Eernisse. All Rights Reserved.