Cladistics
Objectives:
•
•
•
•
•
Understand what the basic components of evolutionary trees are: nodes, branches,
and branch tips.
Understand the difference between anagenesis and cladogenesis, and how those
concepts relate to evolutionary trees.
Understand the difference between a derived trait and an ancestral trait, and between
a synapomorphy and a symplesiomorphy; and, which traits are useful for
determining evolutionary relationships.
Understand what a monophyletic group is and the use of quotation marks for
groupings that are not monophyletic (paraphyletic or polyphyletic).
Be able to build and interpret evolutionary trees.
Exercise 1 – Tree Thinking Challenge
Read Baum et al. (2005) before you come to class! On your desk you will find a
recreation of the phylogenetic tree depicted in Baum et al. (2005) made out of K’nex. Use
this tree to do the following activities and answer the associated questions.
Phylogenetic Tree from Baum et al. (2005):
► On your K’nex tree identify the nodes, branches, and branch tips.
► Based on this tree is a frog more closely related to a fish or a human?
► Identify the node that represents the most recent common ancestor of a frog and a
fish, and identify the node that represents the most recent common ancestor between a
frog and a human. Does this change your answer above? Why or why not?
► Now manipulate your tree without disconnecting any of the pieces so that it looks
like the tree depicted on the right in Baum et al. (2005) and identify the node that
represents the most recent common ancestor between a frog and a human. Did this
change from your previous answer? Why or why not?
Exercise 2 – Building a Cladogram
You and your crew of exobiologists are on a mission to study life in the galaxy. At
each planet you landed on, your job was to gather data for your supercomputer to analyze.
The computer was then able to give the evolutionary relationship for all the life forms in each
planet.
As usual, your ship had reached a new destination, planet BI-140 in the WSU system.
As you approached the planet, you collided with some space debris, lightly damaging your
ship, but severely damaging the Supercomputer. The Supercomputer has shut down all the
non-essential systems and left only the life support system for the crew. The computer is
largely occupied with self-repairs, and this leaves the crew to create a cladogram for life on
this planet. The computer has set an automatic answer system that can answer up to three (3)
questions. Be warned, however, the computer answers only the question you ask (it doesn’t
understand what you “mean.”
To aid in creating a cladogram, the computer was nice enough to give you a list of
instructions to help you before it shut down.
1.
Gather the data you have on the life forms found on this planet.
Robotic rangers have collected and scanned 4 specimens, providing you with
4D holograms that you can manipulate. The actual creatures are held in
quarantine stasis.
2.
List all the characteristics of each creature. The characteristics you
can use are anything you can observe - texture, number of digits, color,
appendages (limb, horn, wing, antenna, etc...), relative size. The
characteristics that you cannot use are absolute size, the inside of the creature
(you do not have the equipment to dissect these creature), and any reference to
the animal on Earth. Remember this is not Earth, these animals might look
superficially similar to Earth creatures, but they are not Earth creatures.
3.
Consider whether these characteristics are defining characters for any
group. Determine if these characters are derived vs. ancestral (i.e. polarize the
traits). Be careful in considering the characters to be plesiomorphies,
symplesiomorphies, apomorphies, synapomorphy or homoplasies.
4
Cladistics
4.
Between each node on the clade, there must be a character state
change.
5.
Use the principle of parsimony to pick the best possible clade.
Once you have the best clade based on the creatures on this planet. You can call the
computer back from shutdown to evaluate the clade. If satisfactory, the computer will then
dispatch the RoboRangers to collect for you 4 new creatures you can use them to test your
clade.
Hints:
1) Utilize the three questions as best as you can to create your clade. Ask questions that
yield the most information, by carefully defining you terms and crafting your
question in a way that maximizes the information you receive. Be sure to list your
questions and the computers answers as part of your write-up.
2) When using size as a comparison, remember to use relative size such as the ratio
between head and body, tail and body, ear and head, front limb to rear limb, etc... Do
not use the actual measurement off of the creatures such as the length of the body,
limbs, etc... as they are merely holograms.
3) Use a strong definition for the characteristics. Consider what is an appendage? what
is a limb? what is the different between the head and the tail? A strong definition will
be useful in differentiate between two different structures.
4) You can use geometric figure to describe a characteristic. For example, triangular
shaped limb, conical shaped spike, quadrilateral shaped head etc...
5) You can’t ignore obvious traits because they confuse things. To eliminate a trait
requires a biological justification.
6) If you use a particular trait to differentiate some organisms, you need to consider that
trait in ALL organisms.
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Cladistics
Sample clades:
Characteristics of each creature.
“Dragon”: Red color, four limbs, spike extended from back to, four digits per front limbs,
three digits per rear limbs, scaly texture, ears, stripe on bottom of body, wings, ear to
head ratio 0.34, head and neck to body ratio 0.44, tail to body ratio 0.42.
“Zebra”: Stripe cover whole body, black white color, smooth body, four limbs, spike
extended from back of the neck, ears, ear to head ratio 0.65, head and neck to body
ratio 0.32, tail to body ratio 0.32
“Sauropod”: Four limbs, four digits on front limbs, three digits on rear limbs, spike extended
from back, spike became elongated, stripe on neck and tail, head and neck to body
ratio 0.36, tail to body ratio 0.41.
Table 1: Defining characters for the creatures.
4
Head Neck Tail
Spikes Ears
Limbs
Stripe on whole Long neck >0.40
body
Dragon
+
+
+
+
+
+
-
+
Zebra
+
+
+
+
+
+
+
-
Sauropod
+
+
+
+
+
-
-
-
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Cladistics
Figure 1: Clade showing the relationship between zebra, dragon, and Sauropod.
Characteristics for each of the additional creature
“Snake”: Uniform body, head.
“Dragonfly”: Two set of wings, six limbs, large eyes, segmented tail
“Mantis”: Ears, six limbs, tail showed to be segmented on to bottom, head and neck to body
ratio 0.10.
“Turtle”: Fusion of digits, dorsal-ventral flattened, spike fused to cover whole body, four
limbs, limbs flattened, reduction of tail, head and neck to body ratio 0.40.
Table 2: Defining characters for the creatures.
Limbs 4 Limbs Head Neck Tail Spikes Ears Stripe on whole body
neck >0.20
Dragon
+
+
+
+
+
+
+
-
+
Zebra
+
+
+
+
+
+
+
+
-
Sauropod +
+
+
+
+
+
-
-
+
Turtle
+
+
+
+
+
+
-
-
+
Snake
-
-
+
-
-
-
-
-
-
Mantis
+
-
+
+
+
-
+
-
-
Dragonfly +
-
+
-
+
-
-
-
-
Cladistics
.
Figure 2: Clade with additional creatures
7
Lab report
You are required to provide two set of tables of defining characters, and two clades;
One for the initial set of creatures, and the other for the original plus additional creatures. In
your lab write up, provide reasons why particular characteristics were preferred over others
for your clade. Which characters would you consider to be synapomorphies or
symplesiomorphies? Identify any homoplastic traits, such as the appearing of wings in two
different lineages in the sample clades.