PHYLOGENY AND SYSTEMATICS
A little bit about taxonomy, classification and phylogeny
Systematics - study of the diversity and relationships of organisms both now and in the past.
Phylogeny - The evolutionary development and history of a species or higher taxonomic grouping of organisms.
Taxonomy - classification, naming and description of taxa.
Methods of establishing phylogenies
Evolutionary systematics - use several fixed levels of a hierarchy, such as kingdom, phylum, class, order, and family.
Evolutionary systematicsPheneticsCladistics
Methods of establishing phylogenies
Phenetics (numerical taxonomy)
Evolutionary systematicsPheneticsCladistics
Robert R. Sokal
C.D. Michener
- based on taking lots of measurements on organisms & then using computer algorithms to assign relationships among them
Problems - ignored ancestral vs. derived characteristics
Methods of establishing phylogenies Evolutionary systematicsPheneticsCladistics
Cladistics - classifies species of organisms into hierarchical monophyletic groups (clades) - based on shared
derived characteristics (or characters)
Willi Hennig
Basic idea - members of a group (clade) share a common evolutionary history
- share unique features that are not present in distant ancestors
Shared derived characteristics - synapomorphies
Important to separate shared derived characters from just shared.
Jelly fish StarfishHuman
Invertebrate ✔✔ ✗
Vertebrate ✗✗ ✔
Water living ✔✔ ✗
Radial symmetry ✔✔ ✗
✔
ASSUMPTIONS OF CLADISTICS
11. Any group of organism are related by common descent from a common ancestor (clade)
2. There is a bifurcating pattern of cladogenesis.
3. Change in characteristics occurs in lineages over time.
Clades
- Can be nested
Clade 1
Clade 2
Clade 3
Methods of establishing phylogenies Evolutionary systematicsPheneticsCladistics
Some definitions
Apomorphy - derived state is a characteristic believed to arisen in a recent common ancestor or a recently evolved feature that appears only in a group of closely related species. The hair of mammals serves to separate them from all other vertebrates
Plesiomorphy -or ancestral character that is present at the base of the tree. For example, the presence of a dorsal nerve chord(shared by all chordates) can be hypothesized to have existed in some common ancestor.
HOW TO CONSTRUCT A CLADOGRAM
1. Choose taxa – must be clades
2. Determine which characters to use- examine each taxon to determine character
states (i.e does the taxon have each character)3. Determine polarity of character state (i.e. is character state original or derived)
5. Build cladogram according to the following rules:
4. Group taxa based on synapomorphies (shared derived characters)
i. All taxa go at endpoints of cladogram
ii. Nodes must have a list of synapomorphies common to all taxa above the node
iii. Synapomorphies should appear only once
Species A
Species B
Species C
Species D
Node 1
Node 2
Node 3
A SAMPLE CLADOGRAM
CAMINACULES
CAMINACULES
CAMINACULES
‘
‘
1. body divided into head and thorax
2. Front appendages
3. 1 pair of front appendages
4. Colour pattern on abdomen
5. Front appendages with ’toes’
6. Toes are rounded
7. Front appendage jointed
TYPES OF CLADOGRAMS
Methods of establishing phylogenies
Monophyletic groups - contain ancestor and all descendant species
Paraphyletic groups - contain ancestor and some but not all descendant species
Polyphyletic groups - contain taxa from two or more different monophyletic groups
Sister group (or outgroup)
CAMINACULES
‘
‘
1. body divided into head and thorax
2. Front appendages
3. 1 pair of front appendages
4. Colour pattern on abdomen
5. Front appendages with ’toes’
6. Toes are rounded
7. Front appendage jointed
Sister group
Monophyletic clades
WHAT KINDS OF CHARACTERS ARE USED?
A. Structural, physiological, behavioural
i. Traits expressed in development
ii. In juveniles
iii. In adults
B. Molecular data
i. Amino acid substitutions in proteins
ii. Genomic genes and gene products
iii. Mitochondrial genes
KINDS OF TRAITS
1. Ancestral ✔
2. Derived ✔
3. Homologous traits- traits derived from same ancestral trait(e.g. limbs in arthropods)
4. Homoplastic (analogous) traits- traits resemble but not due to
common ancestry(e.g. reduction in size to exploit
boundary layer)
Having said all this ……..
There is always some dispute about the relationships in a cladogram. These were all derived from the same data set
ChelicerataCrustacea
Hexapoda
Myriapoda
Chelicerata
Crustacea
Hexapoda
Myriapoda
Chelicerata
Crustacea
Hexapoda
Myriapoda
Chelicerata
MyriapodaCrustacea
Hexapoda
Problems with this phylogeny?
Four Major Evolutionary Trends
1. Evolution of multicellularity and the Metazoa
3. Evolution of the Arthropoda
2. Evolution of triploblastic phyla
4. Evolution of Echinodermata and Urochordata