Download Reading Cladograms

Reading Cladograms
Who is more closely related?
Animals and fungi are more
closely related than either is
to plants.
Determining Evolutionary
Relationships
• Fossils
• Morphology (homologous structures)
– Why can’t we use analogous structures?
• Molecular evidence (DNA, amino acids)
– Uses computer analysis
Cladogram
• Depiction of inferred evolutionary
relationships among a set of species
Cladogram vs. Phylogenetic Tree
• Phylogenetic Trees are a type of cladogram
• Both show relationships between organisms
• Phylo Trees have branches that represent
evolutionary time and amount of change
Important Terms
Relatedness
• Refers to recency of
common ancestry
• Two species are closely
related if their most
recent common ancestor
lived closer to the present
• More distantly related if
their most recent
common ancestor lived in
the distant past
• Think of you + a cousin
• Who is your most recent
common ancestor?
• Think of you + a second
cousin
• Who is your most recent
common ancestor?
• Provides a basis for
assertion that you are
more closely related to
your first than your
second cousin
Evolutionary trees come in a variety of
shapes- do not get distracted by this!
Shared derived characteristics: used to
construct cladograms
• Outgroup:
does not have
anything in
common with
the other
organisms
COMMON ERROR: reading across the tips.
A quick glance at Figure A might suggest that A and B are closely
related because their labels are right next to each other.
This shows that part of the
evolutionary history that was shared by
B and E was NOT shared by A and B.
COMMON ERROR: reading tips as ancestors.
Remember that biologists never view one living
species as the ancestor of another.
COMMON ERROR: not realizing that cladograms
can rotate at the nodes and still show the same
relationships. Think of these as mobiles.
Remember that nodes can rotate.
COMMON ERROR: assuming when a population
evolved or how much gene change occurred in a
linage. You can only do this if it is marked.
- Cannot assume that the wolf
evolved more recently than
the otter
- We DO know that the most
recent common ancestor of
the wolf and otter lived
BEFORE the most recent
common ancestor of the wolf
and coyote
If you are told, or if you see a legend on the tree, THEN
you can say that the length of the lines represents
genetic change or that the line length indicates time.
•
Genetic change: Comparing a homologous gene for development allows the lines
to proportionally represent amounts of genetic change in each lineage. The fruit
fly gene is used as an outgroup.
Branch lengths can represent time:
More Important Terms
• Clade: a group of organisms that includes a
common ancestor and all the descendants of
that ancestor
• Also called a monophyletic group
More Important
Terms
• How to identify a
clade:
– A piece of a larger tree
that can be cut away
from the root with a
single cut
– If you need to 2 cuts to
extract a set of taxa,
the group is nonmonophyletic
Monophyletic, paraphyletic, and polyphyletic groups
An ancestral species
and ALL of its
descendants.
An ancestral species
and some of its
descendants, but not
all of them.
Some of its members
have different
ancestors.
More Important Terms & Ideas
• Every living species has a unique “clade
address”—the list of nested clades to which it
can be assigned
• Trees are hypotheses.
– They can be changed with new discoveries!
• Is a crocodile more similar to a lizard or a bird?
• Is a crocodile more related to a lizard or a bird?
• Which ancestor—the one at node one or node
two—lived closer to the present?
• Note that birds and mammals both are
endotherms, have 4-chambered hearts, and have
insulating skin coverings, but they are not closely
related. This is an example of convergent
evolution.
References
• Baum, D.A., & Offner, S. (2008). Phylogenies and
tree-thinking. The American Biology Teacher, 70(4),
222-229.
• Baum, D.A., DeWitt Smith, S., & Donovan, S.S.
(2005). The tree-thinking challenge. Science,
310(979). doi: 10.1126/science.1117727