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Topic 2: Comparative Method
o Taxonomy, classification, systematics
o Importance of phylogenies
o A closer look at systematics
o Some key concepts
o Parts of a cladogram
o Groups and characters
o Homology and homoplasy
What are taxonomy, classification,
and systematics?
o _________________ – The naming of
organisms and their categorization
o _________________ – The arrangement
of organisms into groups based on trait
similarity (justifying the groups)
o _________________ – The clustering of
groups and organisms based on a unifying
set of principles (evolutionary, or
otherwise)
How would you taxonomize, classify,
and systematize this cutlery?
© www.visualdicationary.com
Taxonomy – naming of taxa
Elk Wrasse Crested Gecko
Japalura Monitor Dog
Boa Goby Fly Solefugid
Photos © K.P. Bergmann
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Classification – categorization based on similarity
Photos © K.P. Bergmann
Mammals
Squamates
Fishes Arthropods
Systematics – clustering of taxa based on
unifying principles
Photos © K.P. Bergmann
What are taxonomy, classification,
and systematics?
o Taxonomy and classification are used
to _______________ things
o Systematics is used to __________
things
Why is systematics important?
“Nothing in biology makes sense except in the light
of evolution.”
- Dobzhansky, 1973
“The task of systematics is the creation of a general
reference system and the investigation of the relations
that extend from it to all other possible and necessary
systems in biology.”
- Hennig, 1950 (1966)
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What aspects of biology are
informed by systematics?
Why is systematics important?
o Provides a
framework for
comparing species
o Accounts for non-
____________ of species
o Species pairs spent
different amount of time
evolving
o __________________
o __________________
Photos © K.P. Bergmann
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Why is systematics important?
Photos © K.P. Bergmann
o Informs about direction of evolution, how species can be compared
2 Species 3 SpeciesThe Evolution of Fur
Some have it, some don’t Fur evolved after scales, scales
are ____________, fur is derived
A Closer Look at Systematics
o Systematics – The clustering of groups or
organisms based on a unifying set of
principles (evolutionary, or otherwise)
o If the system is based on evolution, then it is
_____________________
o These underlying principles are typically
____________ and ______________
Key Concepts of Systematics
o _________________ – The guiding
principle of science, stating that the
simplest explanation that fits the evidence
is most likely the correct explanation
o __________________ – An
evolutionary branching
event, where new taxa arise
from a common ancestor
o ___________________ –
Evolutionary change in a
lineage through time (along
a branch)
Anagenesis
Cladogenesis
Key Concepts of Systematics
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Anatomy of a Cladogram
Operational
Taxonomic Unit (OTU)
Terminal Branch
NodeInternal Branch
Outgroup
Root
Clade: Amphibia
Ingroup
Modified from Pough et al., 2002. Fig. 1-3
Topology
• Sister group
• Dichotomous
• Polytomy
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Types of Groups
o _________________ – A group containing an
ancestor and all of its descendents.
o _________________ – A group containing some, but
not all, of its descendents.
o _________________ – A group containing taxa that
lack a common ancestor.
Types of Characters
o A character can comprise any quality or
quantity that can be compared between the
taxa under study
o _______________
o Morphometrics
o _______________
o Ecology
o _______________
o Sonography
o __________________
o Allozymes
o Karyotype
o Protein structure
o Etc.
Types of Characters
o _____________________ – Alternate forms of a
character in the taxa being studied
• These represent the variation that is analyzed by systematic
methods
© K.P. Bergmann
e.g. Pupil
shape
Round Slitted Slit & pin-hole
Types of Characters
o ______________ – An ancestral feature,
common to taxa in the ingroup and outgroup
o ______________ – A shared derived feature,
common to clusters of taxa of the ingroup
o ______________ – A unique derived feature,
diagnostic of a taxon (OTU) in the ingroup
Sauropsids (‘Reptilia’ &
Aves)
Diapsida (Archosauria &
Lepidosauria)
Aves (Birds)
Plesiomorphy
Synapomorphy
Autapomorphy
Amniotic eggSingle centrale
bone in ankle
Of Diapsida:
2 temporal
foramina in skull
Of Aves:
Feathers
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Single centrale
bone in ankle
2 temporal
foramina in
skull
2 temporal
foramina in
skull
Feathers
Types of Characters
Types of Characters
Taxon
Frog 0 0 0 0 0 0 0 0 1
Turtles 1 0 0 0 1 0 0 0 1
Lepidosaurs 1 0 1 1 0 0 1 0 1
Crocodylians 1 0 1 1 0 1 0 0 1
Birds 1 0 1 1 0 1 0 1 1
Monotremes 1 1 2 0 0 0 0 0 1
Marsupials 1 1 0 0 0 0 0 0 1
Eutherians 1 1 0 0 0 0 0 0 1
Modified from Liem et al 2001, Focus 1-2, Table A
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Types of Characters
Amniotic Egg
Single Centrale
Trunk in Shell
Fenestra in jaw
Diapsid Skull
Transverse Cloaca
Feathers
Digits on Limbs
Hair
Liem et al. 2001, Focus 1-2, Fig A
Frog
Turtle
Lepidosaur
Crocodilian
Bird
Monotreme
Marsupial
Eutherian
Digits on Limbs
Homology and Homoplasy
o _______________ – The fundamental similarity
of a character between taxa within a group
• Structures compared between taxa must be of the
same evolutionary origin (comparing apples to apples)
o Homology – comparing apples to apples
Modified from Linzey, 2001
Homology and Homoplasy
Homology and Homoplasy
_______________________ is when homologous structures occur
within an individual Examples:
• Vertebrae
• Feathers
• Hair
• Scales
• Teeth
• Front & Hind limbs
Examples:
• Testes & ovaries
• Glans penis & glans clitoridis
• Scrotum & labiae majorae
_______________ is when sexually dimorphic homologous structures
occur in males and females
o ____________________ – The similarity of a character or structure between taxa due to convergence
o Homoplasy can obscure relationships between taxa
by leading to erroneous:
o Statements of homology
o Coding of character states
o Phylogeny reconstruction
Homology and Homoplasy
o Homoplasy is thought to occur due to:o _________________________ in different groups
o _________________ on the direction of evolution
Homology and Homoplasy
Photos © PJB
Endothermy:
• Aves
• Mammalia
Allows sustained activity and
independence of metabolism
from environmental
temperatures
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Homology and
Homoplasy
Photos © PJB
Endothermy in Aves and
Mammalia has evolved
twice independently
Frog
Turtle
Lepidosaur
Crocodilian
Bird
Monotreme
Marsupial
Eutherian
Liem et al. 2001, Focus 1-2, Fig A
o Homoplasy is thought to occur due to:• Similar selective pressures in different groups
• Constraint on the direction of evolution
Homology and Homoplasy
Modified from Linzey, 2001; Liem et al. 2001, Fig 1-9C
Homology or Homoplasy?
Aves Chiroptera Pterosauria