Vertebrate Zoology
Lab 1 – Cladistic Analysis
Terms to know
Taxon – A species or group of organisms that is given a name in a formal naming
system.
Cladogram – A diagram that illustrates hypotheses about evolutionary
relationships. The key characteristic that separates cladograms from
other diagrams that illustrate systematic information about taxa is that
cladograms focus explicitly on ancestor-descendant relationships.
Monophyletic group – A group of species that contains a common ancestral
species and all of its descendent taxa. Also referred to as a clade.
Characters – Heritable phenotypic or genotypic traits used in cladistic analysis.
Homologous characters – Characters that share a common evolutionary origin.
Convergent characters – Characters that appear functionally or structurally
similar, but do not share a common evolutionary origin.
Ancestral characters – Older characters that are present in most, if not all
members of a clade back to the most recent common ancestor (also
known as a plesiomorphy).
Derived characters – More recent characters (compared to ancestral characters)
that may not be shared by all members of a large, inclusive clade (A
derived character shared among two or more taxa is known as a
synapomorphy).
Homoplasy – Character reversal (reversion to the derived state) or characters
that have developed more than once in a lineage (=convergence)
Character state polarity analysis – A process used to determine ancestral and
derived characters using outgroups and ingroups.
Ingroups – The group of taxa being investigated. It is either assumed or has
been demonstrated that the members of the ingroup represent a clade.
Outgroup – Taxa (or taxon) most closely related to the members of the ingroup.
Node – A branching point in a cladogram. Each node represents a hypothetical
ancestral species.
Internode – The line between two nodes or between a node and a terminal taxa.
Internodes represent an evolutionary sequence leading to the next node
or a terminal taxa.
Steps in Phylogenetic Analysis
1.
2.
3.
4.
5.
6.
7.
8.
Select an ingroup to study.
Designate terminal taxa in your ingroup.
Select outgroups.
Identify and define character states within the ingroup. Useful characters
must have two or more alternative states in the ingroup to be useful in the
analysis.
Determine ancestral and derived character states by comparing the
ingroup to the outgroup. Character states present in both the ingroup and
the outgroup are assumed to be ancestral.
Working with one character, group the taxa in the ingroup that share the
derived state(s). Construct a cladogram that represents these
relationships.
Repeat step 6 until all characters states have been represented on your
final cladogram.
Assess whether any characters represent homplasies.
Practice, practice, practice
In lab, I will have a figure with a number of hypothetical frog species. We will
use these frogs to perform a cladistic analysis. I have listed the characters for
these frogs below and provided a table that can be used to quantify the
character states present or absent in each taxon. Please bring this with you to
lab on Tuesday.
Character States
Character
1. Claws
2. Chin Hair
3. Horn
4. Lateral Fold
5. Tympanum
6. Spots
7. Tail
8. Spikes on Tail
9. Digits on Forelimb
10.Nostril
Ingroup Character State(s)
Present or Absent
Present or Absent
Present or Absent
Present or Absent
Present or Absent
Present or Absent
Present
Present or Absent
4 or 5
Present or Absent
Character
Claws
Chin Hair
Horn
Tail
Spikes
Digits
Spots
Tympanum
Lateral Fold
Nostril
Outgroup 1
Outgroup 2
Species A
Species B
Species B
Species D
Species E
Species F