Phylogenetic Analysis – Part 2

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Outline Why do we do phylogenetics

(cladistics)?

How do we build a tree?

Do we believe the tree?

Applications of phylogenetics (cladistics)

Why do we do phylogenetic analyses?

Evolutionary interpretation of relationships between organisms.

More reliable method of ascertaining the best hypothesis for how the organisms diverged from common ancestors.

Provides a better rationale for circumscribing taxonomic groups.

Enables testing of multiple hypotheses of relationships based on specific character transformations.

Willi Hennig(1913-1976)

German Entomologist (Diptera)

Warren Herbert Wagner(1920-2000)

American Botanist (Pteridophytes)

How do we build a tree? Data: Types of characters

Look for synapomorphies

Assemble the hierarchy of synapomorphies according to the principle of parsimony

Parsimony The idea that the simplest hypothesis is the

best explanation given the assumptions of the analysis.

Also known as Occam’s Razor In systematics, we look for the shortest trees

in any given phylogenetic analysis (maximum parsimony)…

…even though we know that evolution doesn’t necessarily proceed in a parsimonious manner.

Data: type of charactersCharacter Selection:

Must study each individual character to assure comparisons of homologous characters, and interpretation of the direction of character state transformations.

Some sources of characters: 1) morphology (external structure) or anatomy (internal structure)

2) biochemical (photosynthetic pathway, pigmentation pathways, etc.)

3) chromosome numbers

4) nucleotide sequence data ( a t c g )

Character states Presence vs. absence (0 or 1): always binary

(two states) Other binary characters (0 or 1): yellow vs.

white flowers Multi-state characters (3 or more states): e.g.,

yellow, white, or pink flowers or a t g c for nucleotide sequence data

For binary or multi-state characters, can hypothesize the direction of evolutionary change (transformation series)

But how to hypothesize direction?

We usually have some working knowledge of the group under study (ingroup).

We choose a group thought to be closely related to the ingroup to serve as the outgroup as a basis of comparison.

Character states in the outgroup are assumed to be ancestral (plesiomorphic).

This allows us to establish the direction of character state transformation.

Phylogenetics or Cladistics

Oppositifolia

We need an outgroup, so wechoose the closely related genus Alternifolia.

Cladistics

Alternifolia

Oppositifolia

Cladistics: characters

1. Leaf arrangement: alternate (0) or opposite (1)

2. Leaf midrib: white (0) or green (1)3. Leaf apex: rounded (0) or spiny (1)4. Sepals: present (0) or absent (1)5. Flower color: yellow (0) or blue (1)6. Subtending floral bracts: absent (0) or

present (1)

Cladistics: characters

1. Leaf arrangement: alternate (0) or opposite (1)

2. Leaf midrib: white (0) or green (1)3. Leaf apex: rounded (0) or spiny (1)4. Sepals: present (0) or absent (1)5. Flower color: yellow (0) or blue (1)6. Subtending floral bracts: absent (0) or

present (1)

Cladistics: matrixCharacter 1 2 3 4 5 6Species 1 1 1 0 1 1 0Species 2 1 0 0 1 0 0Species 3 1 1 0 1 0 1Species 4 1 1 0 1 1 0Species 5 1 1 0 1 0 1Species 6 1 1 1 1 1 0Species 7 0 0 0 0 0 0Species 8 0 0 1 0 0 0

Outgroup

Cladistics7 8 2 4 3 5 6 1

Leaves opposite

Sepals absent

Homoplasy?Polymorphism in the outgroup

purple fringe

sepals absent

floral bractspresent

midrib green

midrib white

Length = 5 steps

ancestral

purple fringe

sepals absent

floral bractspresent

midrib green

midrib white

spiny leaftip

spiny leaf tip

Length = 7 steps

ancestral

Do we believe the tree? Various programs to generate trees. Various measures of statistical support

for the clades and for the characters. Can quantify the effects of homoplasy. Can test alternate arrangements to

examine the number of steps involved.

Phylogenetic Methodology1. Selection of taxa to study - Individuals, populations,

species, etc. identified as the units of comparison. One or more related groups (outgroups) necessary to “root” the trees.

2. The units under study described for as many characters as possible for which homology can be demonstrated or reasonably assumed. Character states assigned based on variation among the taxa in the ingroup and outgroup(s) and a priori hypotheses of the evolutionary direction of changes undergone by the character (character state polarization) are generated based on outgroup comparison.

3. A data matrix is assembled by scoring all taxa for all characters (ideally).

Phylogenetic Methodology(continued)

4. Using various analytical principles (such as maximum parsimony), a cladogram is constructed using synapomorphic character state changes to determine the tree topology. Systematists are seeking to define monophyletic groups (= clades).

5. Evaluation of the statistical confidence in how robustly the data support the grouping of organisms into clades is done next.

6. The subsequent grouping and ranking of the organisms in the resulting clades is then applied to classification systems.

Applications of phylogenetics

Classification

Biogeography

Many other possibilities (e.g., disease tracking, gene annotation)

Cladistics and Classification- - Classifications based on tree topologies…Is it the best tree? Will new

data and new tree topologies necessitate complete overhaul of classification?

- Remember, systematists would like to define monophyletic groups (clades). Is this always possible? Must we accept paraphyletic groups?

- Classifications must also be useful for communication. Do the groups truly represent the best evolutionary hypothesis, and yet are the classifications useful?

Biogeography: Fig. 1.9 from the text

Phylogenetic Analysis – Part 2

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