Building Trees: Introducing evolutionary concepts by exploring Crassulaceae
phylogeny and biogeography
Pre-lab questions – Part 1
Note: Review the Phylogeny PowerPoint and answer these questions.
1. In a phylogenetic or evolutionary tree, how is relatedness defined, i.e. what is used to
determine relatedness?
Relatedness is about descent from a common ancestor and not about character
similarity. Some distantly related organisms can share similar characteristics, e.g.
lizard-like body form of lizards and crocodiles. Some more closely related organisms
may differ in many characteristics, e.g. crocodile and bird.
2. What is a monophyletic clade? In the tree below indicate what you would circle to
define at least 3 monophyletic clades.
A clade includes all of the descendants of a common ancestor and that common
ancestor. It can be cut off from the rest of a phylogenetic tree with a single cut.
Students could circle a number of different monophyletic clades in this tree. E.g. E and
D; E, D and C; E, D, C and B; and A, E, D, C and B.
3. What is a polytomy and what does it imply about evolutionary relationships?
A polytomy is a branch showing more than two descendant species or taxa from a
lineage. While this may imply multiple evolutionary events in most trees, polytomies
are used to imply phylogenetic uncertainty.
4. What is the principle of parsimony and how is it used in developing evolutionary
relationships among different taxa?
According to D. Baum in the Tree Thinking article, “The parsimony criterion states that
the most plausible mapping of a character onto a tree is that which invokes the fewest
changes. [In developing evolutionary relationships] This does not guarantee that this is
what actually happened, but simply means that in the absence of contrary evidence, this
is generally the best bet for the true evolutionary history.” When inferring phylogenies,
the parsimony criterion states that we should favor the tree that can explain all the
data with the lowest number of evolutionary changes.
Building Trees: Introducing evolutionary concepts by exploring Crassulaceae
phylogeny and biogeography
5. What is a most recent common ancestor (MRCA) and how is this information used in
understanding phylogenetic/evolutionary relationships among organisms? For
example, in the following diagram:
a) What is the MRCA of the frog and the lizard?
The MRCA is circled in red (below).
b) What is the MRCA of the frog and the human?
The same node - the MRCA is circled in red.
c) Is the frog more closely related to the lizard or the human?
Since the lizard and human share the same MCRA with the frog neither is more closely
related than the other. However, the lizard shares a more recent common ancestor with
the human than with the frog – Blue circle.
Building Trees: Introducing evolutionary concepts by exploring Crassulaceae
phylogeny and biogeography
6. Examine the two sets of phylogenetic trees below. For each pair, indicate whether
the two trees contain the same information.
The first pair contains the same relationships – some are simply rotated at the nodes. The
second pair contains different relationships. In the one on the left A is more closely related
to B than C and in the one on the right A is more closely related to C than to B.
SET 1
SET 2
O
C
B
A
O
B
A
C