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