Phylogenetic Trees
Name __________________________
23 October 2013
Name __________________________
Name __________________________
Lizards
Ray-finned fish
Mammals
Snakes
Amphibians
1
1
1
1
1
1
0
1
0*
1
1
0
1
1
0
0
0
1
0
0
Scales with
keratin
Fur,
lactation
Amniotic
egg
Limbs
Internal
bone
The table below summarizes key traits found in some major groups of animals that have vertebrae, or backbones. A
“1” indicates that the trait is present; a “0” indicates that it is absent. Assume that each of the traits is homologous in
the groups listed—for example, that the limbs of lizards and mammals are derived from limbs found in a common
ancestor.
1
0
0
1
0
*Note that some snakes have vestigial hindlimbs as embryos or as adults.
Use these data to estimate the phylogeny of the groups. Here’s one way of getting started:
1. Note that only lizards and snakes have scales with
keratin—meaning that they should be found next to
each other on the tree, joined by a common
ancestor that had scales with keratin. In the space to
the right, start your tree by connecting lizards and
snakes with a u-shaped bracket (
). Add a
“scales” label to the left-most branch, to indicate
where the trait evolved.
Sharks and rays
Scales Limbs lost
w/keratin
Lizards
Amniotic
egg
Limbs
2. Now look at the “Amniotic egg” column. Note that
only lizards, mammals, and snakes have this trait—
the other groups do not. Thus, the amniotic egg is a
synapomorphy that defines the lizards, snakes, and
mammals as a monophyletic group. Use this
Internal
information to add mammals to your tree. Add an
bone
“amniotic egg” label to show where the trait evolved.
Snakes
Fur,
lactation
Mammals
Amphibians
Ray-finned fish
3. Examine the table again, and find synapomorphies
that will allow you to add ray-finned fish and
amphibians to your tree. Label where each
synapomorphy evolved.
4. When you have placed all 5 groups, add a 6th branch
called “Sharks and rays” as an outgroup.
5. Which group is most closely related to the amphibians: lizards or sharks and rays? (This is the same as asking
which of the two groups shares a more recent common ancestor with amphibians.) Explain your reasoning.
Lizards, because they share a common ancestor with amphibians at the node marked with a fat arrow.
Amphibians and sharks and rays share a common ancestor at the node marked with a skinny arrow.
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6. Which group is most closely related to the sharks and rays: Lizards or ray-finned fish? (This is the same as
asking which of the two groups shares a more recent common ancestor with sharks and rays.) Explain your
reasoning.
They are equally closely related to sharks and rays, because they both share a common ancestor with
sharks and rays at the node marked with a skinny arrow.
7. According to your tree, are mammals higher than amphibians? Explain why or why not.
No—they are not “more evolved” or “more advanced” or more of anything else that could be construed as
higher. They simply have some derived traits (synapomorphies) that amphibians do not. (Likewise,
amphibians have derived traits that mammals do not.) Amphibians and mammals are both tips on the tree,
not steps on a ladder. Biologically, “higher” is meaningless.
8. Snakes don’t have limbs. Do they really belong in a monophyletic group with vertebrates that do have limbs?
Explain why or why not. (If appropriate, add a “limbs lost” label to your tree.)
Yes—there is strong evidence, including the presence of vestigial traits, supporting the hypothesis that
their ancestors had limbs. During the evolution of snakes, limbs were lost.
9. Relative to the common ancestor of all the bony vertebrates (meaning, the common ancestor of all of the
groups on the tree except for sharks and rays), which of the groups on your tree has been evolving for the
longest amount of time? Explain your reasoning.
All of the species living today have been evolving for the same length of time since their common ancestor
existed.
10. Sharks and rays don’t have bony skeletons—they have cartilaginous skeletons that also occurred in their
ancestors. Is either of the following statements correct? Explain why or why not.
a) Sharks and rays are a basal (you could also say “primitive” or “ancestral”) group relative to the vertebrates.
b) Cartilaginous skeletons are a basal (you could also say “primitive” or “ancestral”) trait relative to the bony
skeletons of vertebrates.
Statement b) is correct; a) is not. The sharks and rays living today are descended from ancestors with
cartilaginous skeletons that lived long ago. But those ancestors were not the same as today’s sharks and
rays. So sharks and rays may have some primitive/basal/ancestral traits—meaning, traits retained from that
ancestor*—but the shark and ray species living today are not primitive (or basal, or ancestral).
*This is true of all species. Humans retain ancestral traits such as the vestige of a tail, genes comprised of
DNA, presence of limbs, and so on.
********* Please turn the completed exercise in to your T.A. (and make sure that your names are legible!)
*********
****************** We’ll send you an answer sheet to the exercise on Monday, via email ***************
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