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Saturday Study Session
AP Biology
CLADISTICS
Student Packet
SUMMARY
PHYLOGENETIC TREES AND CLADOGRAMS ARE MODELS OF EVOLUTIONARY HISTORY THAT CAN BE TESTED
• Phylogeny is the history of descent of organisms from their common ancestor.
• Phylogenetic trees/cladograms are constructed by employing computer programs that measure and model
relatedness among organisms.
• When constructing phylogenies, scientists consider
 fossils record,
 molecular similarities such as DNA or amino acid sequences,
 comparisons of embryology, metabolic pathways, reproductive strategies, behavioral traits,
 biogeography and
 homologous structures.
•
•
Homologous structures are any features shared by two or more species that are inherited from a common
ancestor. Vestigial structures are examples of homologous structures; they are remnants of functional
structures and when compared to fossils, they provide evidence for evolution.
On the other hand, analogous structures should not be used in establishing phylogenies. Analogous structures
are those that are similar in structure but are not inherited from a common ancestor. Analogous structures
evolve through convergent evolution, the independent evolution of similar features in different lineages, as a
result of similar selection pressures. The placental mole and the southern Australian mole are both burrowing
mammals, yet are not closely related.
Bird and bat wings are analogous because they
have separate evolutionary origins, but they
evolved to serve the same function. Birds and bats
did not inherit wings from a common ancestor
with wings.
However, forelimbs of birds and bats are
homologous because they inherited forelimbs
from a common ancestor with forelimbs.
INTERPRETING PHYLOGENETIC TREES/CLADOGRAMS
• Groups (taxa) of evolutionary related organisms are represented as branches in cladograms.
• The tips of the tree represent groups of descendent taxa and nodes (branching points) represent the common
ancestors of those descendants. When a lineage splits into two branches, a speciation event occurred.
• Phylogenetic trees/cladograms can represent traits that are either derived or lost due to evolution. The original
shared trait is termed the ancestral trait and the trait found in the newly evolved organism is called the derived
trait. An example of a lost trait is lack of legs in sea mammals.
• Sister taxa are groups or organisms that share an immediate common ancestor and are very closely related.
• Outgroup is a group/taxon that is less closely related to the other organisms.
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Saturday Study Session
AP Biology
•
A clade (a.k.a. monophyletic group) is any taxon that consists of all the evolutionary descendants of a common
ancestor. All clades are subsets of larger clades, with all life as the most inclusive taxon.
•
Every node in a phylogenetic tree should give rise to two lineages. If more than two lineages are shown, it
indicates an unresolved pattern of divergence (polytomy).
Branches can rotate and still represent the same phylogeny. The two phylogenetic trees below illustrate the
same evolutionary relationships. (The vertical branches have been rotated).
•
•
•
•
If there are several hypotheses about relatedness of organisms, the principle of parsimony is used: the simplest
explanation with fewer evolutionary events is more likely to have occurred.
Phylogenetic trees may include estimates of times of divergence of lineages determined by the molecular clock
analysis. The molecular clock hypothesis states that among closely related species, a given gene usually evolves
at a reasonably constant rate. The number of mutation events in a gene can be used to predict times of
evolutionary divergence.
Phylogenetic trees are constantly being revised based on the biological data used, new mathematical ideas and
emerging knowledge.
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Saturday Study Session
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MULTIPLE CHOICE QUESTIONS
Questions 1-2
The cladograms below show possible evolutionary relationships among four main groups of plants: nonvascular plants
such as mosses, seedless vascular plants such as ferns, naked seed vascular plants such as pines and flowering plants
such as oaks.
1. What is the outgroup in the cladograms?
a.
b.
c.
d.
Mosses
Ferns
Pines
Oaks
2. Based on the information shown in tree (b), which plants have a vascular system and seeds?
a. Mosses and ferns
b. Ferns and pines
c. Pines and oaks
d. Oaks
3. The cladogram below shows the evolutionary relationships in vertebrates. Which if the four letters corresponds
to evolution of lungs?
a.
b.
c.
d.
A
B
C
D
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Saturday Study Session
AP Biology
Question 4-5
The heart is a muscular organ which pumps blood throughout the circulatory system. In vertebrates, there are three
types of hearts: a two-chambered heart (consisting of one atrium and one ventricle), a three-chambered heart
(consisting of two atria and one ventricle) and a four-chambered heart (consisting of two atria and two ventricles). The
cladogram below depicts the evolution of the heart.
4. Which of the following statements is supported by the cladogram above?
a. Zebrafish and lampreys have identical circulatory mechanisms.
b. The heart of clawed frog is more similar to the human heart than to the heart of pufferfish.
c. Birds have a three-chambered heart.
d. The circulatory system of zebrafish is more closely related to the circulatory system of lampreys than to the
circulatory system of green anole lizards.
5. Which of the following is an incorrect interpretation of the information presented in the cladogram?
a. Homeostatic control systems, such as the circulatory system, reflect common ancestry.
b. The hearts of animals evolved new mechanisms due to adaptations to different environments.
c. Organisms share conserved core processes that evolved and are widely distributed today.
d. Several mammals, such as mouse and opossum, evolved circulatory mechanisms independently from other
mammals.
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6. The table below depicts characters of several animal species.
Character
Hair
Carnivorous
Retractable Claws
Mammary glands
Ability to purr
Backbone
A - Turtle
–
–
–
–
–
+
B - Horse
+
–
–
+
–
+
C - Wolf
+
+
–
+
–
+
D - Cat
+
+
+
+
+
+
E - Leopard
+
+
+
+
–
+
Which of the following cladograms is the most accurate representation of the information in the table?
a.
b.
c.
d.
Cladogram 1
Cladogram 2
Cladogram 3
Cladogram 4
7. The North American flying squirrel can be found at night gliding among the tree-tops and gathering insects for
food. The Australian sugar glider essentially leads the same lifestyle. If these two species of squirrels were
placed side by side, they would look almost the same. However, the North American flying squirrel is a placental
mammal while the Australian sugar glider is a marsupial mammal. How is it possible that these organisms look
identical?
a. The two species of squirrels are displaying adaptive radiation. When Pangaea broke apart, the squirrels
moved away from each other and adapted to different climates.
b. Flying squirrels and sugar gliders have undergone sympatric speciation during Pangea.
c. This is an example of convergent evolution. The two species appear to be one because they have evolved in
similar environments and similar traits were favored for survival.
d. These two species share a distant common ancestor. The process of divergent evolution forced them to
evolve into two different species over time.
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Saturday Study Session
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MATH GRID IN
1. The molecular clock hypothesis states that among closely related species, a
given gene usually evolves at a reasonably constant rate. The number of
mutation events in a gene can be used to predict times of evolutionary
divergence.
Cytochrome c is an electron carrier of the electron transport chain located in
the inner mitochondrial membrane. On average, this protein mutates every
17.7 million years. In mammals, cytochrome c contains 15 different amino acids
than the reptilian cytochrome c. Using the molecular clock hypothesis,
calculate how many million years ago mammals and reptiles diverged. Give
your answer to the nearest whole number.
SHORT FREE RESPONSE QUESTIONS
1. Mammalian milk contains antibodies that are produced by the mother’s immune system and passed to offspring
during feeding. Mammalian milk also contains a sugar (lactose) and may contain proteins (protein A, protein B,
and casein), as indicated in the table.
MILK COMPONENTS IN DIFFERENT MAMMALS
Character
Cat
Cow
Horse
Human
Lactose
+
+
+
+
Protein A
+
+
+
+
Protein B
–
+
+
–
Casein
–
+
+
–
+ indicates the presence of the character and – indicates the absence of a character
Pig
+
+
+
+
Using the data in the table, construct a cladogram on the template provided to indicate the most likely
evolutionary relationships among the different mammals. Indicate on the cladogram where each of the
characters most likely arose in the evolutionary process, and justify the placement of the characters on the
cladogram.
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Saturday Study Session
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2. Based on the data in the table below, draw a phylogenetic tree that reflects the evolutionary relationships
of the organisms based on the differences in their cytochrome c amino-acid sequences and explain the
relationships of the organisms. Identify which organism is most closely related to the chicken and explain
your choice.
THE NUMBER OF AMINO ACID DIFFERENCES IN CYTOCHROME C AMONG VARIOUS ORGANISMS
Horse
Donkey
Chicken
Penguin
Snake
Horse
0
1
11
13
21
Donkey
0
10
12
20
Chicken
0
3
18
Penguin
0
17
Snake
0
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Saturday Study Session
AP Biology
LONG FREE RESPONSE QUESTION
Phylogeny reflects the evolutionary history of organisms.
a) Explain THREE methods that have been used to investigate the phylogeny of organisms. Describe a strength or
weakness of each method.
b) The two phylogenetic trees represent the relationship of whales to six other mammals. All of the organisms
shown have a pulley-shaped astragalus bone in the ankle except for the whale.
i.
For each tree, describe a monophyletic group, the closest relative to the whale, and the point at which the
pulley astragalus was lost or gained.
ii.
Based on the principle of parsimony (the simplest explanation is the best) and the genomic information in
the table shown, identify which tree is the best representation of the evolutionary relationship of these
animals, and justify your answer.
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Saturday Study Session
AP Biology
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