Chapter 26: Phylogeny and the Tree of Life
Phylogeny
The evolutionary history of a
species
Systematics
The study/process of tracing
the phylogeny of organisms
to construct an evolutionary
“Big Picture” relationships
Tools of Study: Taxonomy and Cladograms
A. Tool #1 Taxonomy
Taxonomic categories are incorporated into cladograms
(Phylocode) Taxonomic systems totally based on evolutionary relationships
Phylogenetic Tree
Note proper form:
1. Genus Capitalized;
species small case,
2.Both in italics or
underlined
Mustela
frenata
Branch Point
(node)
Taxonomy Lingo:
1. Taxon:
3. Sister taxa:
4. Rooted:
5. Polytomy:
Branch length
can represent
time or genetic
change!
The taxonomic group at any level
2. Branch Point / Node:
polytomy
Sister
Taxa
Taxa (plural)
The divergence of 2 taxa from a common ancestor
Taxa that share the most recent common ancestor
Oldest branch to the left
A branch of more than 2 immediate descendents showing unclear evolutionary
6. Monophyletic taxon Contains all members of the same
“one tribe”
ancestry. This is the only valid taxon
derived from a cladogram
leopard
cat
Felidea
7. Paraphyletic taxon
Wolf Dog Horse Zebra
Canidea Perisodactyla
Invalid taxon because it consists of a
group of some with similar ancestry
(I&K), but not all (J plus A and all its
descendents) of the ancestors or
descendants
Carnivora
Mammalia
leapard cat Wolf Dog
Horse
Zebra
8. Polyphyletic taxon Invalid taxon because it contains
members that lack a common ancestor
Cladograms & Proper Taxonomic Categories (Taxon)
Felidea
Canidea Perisodactyla
Carnivora
Mammalia
leopard
cat
Felidea
Wolf Dog Horse
Canidea
Carnivora
Mammalia
Zebra
Perisodactyla
Tool #2 The Cladogram
A phylogenic tree with a series of 2 way branches that present the
divergence of related organisms through a common ancestor
Salamander
Lancelet
Lamprey
Outgroup
Tuna
Turtle
Leopard
Ingroup
Hair
Clade
Amniotic Egg
Four walking
legs
Jaws
Cladogram Lingo:
1. Clade
Vertebral Column
A branch in the cladogram
2. Homologous Characters
Shared primitive character
Traits (characters) that are of similar ancestry
3. Shared primitive character
4. Shared derived character
5. Outgroup
Shared derived
characters
The trait that is found in all members of the cladogram
The trait found in the continuing branch of the cladogram
The comparative organism that does not have the shared primitive character
6. Ingroup
The members of the cladogram that all have the shared primitive character
The assumption that the simplest arrangement with the fewest required
7. Maximum
Parsimony (Occam’s changes is the best explanation. Assumptions of parsimony include:
Razor)
a) It is more likely that a unique feature evolves only once and
passed down as opposed to evolving twice in different groups
Exceptions: Birds - 4 chambered heart
Protists - development of multicellularity
b) More complex structures evolve from simpler
Exception: Parasites
#3 – Organism’s genome contains in evolutionary history
A. Different genes evolve at different rates
1. DNA coding for ribosomal RNA (rRNA) changes slowly, best
used when taxa diverged 100s mya
2. Mitochondrial DNA (mtDNA) evolves rapidly,
best used when exploring recent events
B. Gene duplicationUnequal crossing over during Prophase I that generates
new genetic material, leading to molecular evolution
~Forms gene families: groups of related genes within an organism’s
genome
Ex. Human α-globin and β-globin genes (proteins on surface of RBCs)
1. Orthologous genes - homologous genes found in different
species because of SPECIATION
2. Paralogous genes - formed from gene duplication, found in more
than one copy in the SAME GENOME
Gene Duplication due to
unequal crossing over
Slide 5
Model for Evolution of Human Globin Genes
Slide 18
Slide 5
Slide 5
Applying the Principle of Parsimony
Slide 2
Parsimony and the Analogy-verse-Homology Pitfall
Traditional Cladogram
“The Mammal- Bird Clade”
New Cladogram
“The Bird-Reptile Clade”
New evidence from other derived characters place birds with reptiles
Four chambered heart evolved twice thus it is an analogous character
not a homologous character
Slide 2
Slide 2
Example of Polytomy
Normal (dichotomous)
phylogeny
Polytomy in phylogeny
-At all branch points
there are two immediate
descendants
-There’s a polytomy from which
dragonflies, mayflies, & beetles
arise, indicating that the
relationships between these
three lineages is not yet clear
Branch Length Representing
Genetic Change
Branch Length Indicating Time
Slide 3