Lecture 10 evolution classification cont d
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Lecture 10: Evolution & Classification cont’d. Species Classification: Phenetic: physical attributes, numerical taxonomy Cladistic (Phylogenetic): e volutionary relationships Evolutionary: synthesis of the two Reflect Philosophical Differences. Phenetic Classification. “Like with like”

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Lecture 10 evolution classification cont d l.jpg
Lecture 10: Evolution & Classification cont’d

Species Classification:

  • Phenetic: physical attributes, numerical taxonomy

  • Cladistic (Phylogenetic): evolutionary relationships

  • Evolutionary: synthesis of the two

    Reflect Philosophical Differences


Phenetic classification l.jpg
Phenetic Classification

  • “Like with like”

  • Use many characters to define overall similarity

  • Linnaean

    • Before Darwin so not based on Evolution

      (but may reflect history)

  • Current: reaction to uncertainty of cladistics

  • Problem: uses all types of characters (analogies, ancestral & derived homologies)


Steps l.jpg
Steps

1) identify taxa to be considered

2) choose characters (independent, “unit”)

3) construct character matrix for each taxon:

4) use mathematical formula to describe degree of similarity for each taxon:

e.g. simple matching coefficient

# matches

total # of characters

S =


Slide4 l.jpg

5) construct matrix with pairwise S values

6) use clustering technique to produce a dendrogram

e.g. UPGMA (Unweighted Pair Group Method with Arithmetic Averaging)

or Neighbour-joining

Unweighted/Equal weighting = all characters given equal consideration

unweighting is a type of weighting!

may introduce bias…..


Example l.jpg
Example

Character Matrix

S-value Matrix


Joining clusters l.jpg
Joining Clusters

Closest: A&D = 0.7

2nd Closest B&C = 0.5

When does A&D join B&C ?

(A&B) + (A&C) + (D&B) + (D&C) 4

= (0.3 + 0.4 + 0.4 + 0.3)/4 = 0.35


Problems l.jpg
Problems

  • Different methods or characters = different dendrograms

  • If used all characteristics would = natural classification (Impossible!)

  • dendrogram = phylogeny if differences between taxa proportional to time elapsed since common ancestor


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Unfortunately…

  • Mosaic Evolution: differences in rate of change of characters in a lineage

    2. Homoplasy: shared characters not in common ancestor (analogy)


Mosaic evolution l.jpg
Mosaic Evolution

  • ancestral & derived characters differ among lineages

  • different characters evolve at different rates


Why retained l.jpg
Why retained?

  • Developmental Canalization

  • Character change requires change

    in developmental program (rare)

    B) General Adaptations

  • Useful in large number of ecological contexts

    e.g. Rodentia - incisors conserved

    - legs evolved rapidly


Homoplasy l.jpg
Homoplasy

 # characters used,  chance of homoplasy

  • Convergent Evolution

  • Similar phenotypic response to similar ecological conditions

  • Different developmental pathways


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B. Parallel Evolution

  • Same developmental pathway, independent evolution

    e.g. elongated body of burrowing salamanders evolved independently :

    increased size of some vertebrae : convergence

    increased number of vertebrae : parallelism


Slide13 l.jpg

C. Evolutionary Reversal

  • degeneration of complex structure

  • looks primitive, actually derived

  • e.g. Winglessness in Fleas & Lice

    • 2 different winged ancestors

      Dollo’s law : complex structures that are lost are unlikely to be regained

      Exceptions: snake eyes, molars in some felids


Frogs with teeth l.jpg
Frogs with Teeth?

  • Reversals & Parallelism common because of potentialities (bias) of developmental systems

  • Frogs lost teeth in lower jaw in the Jurassic

  • Teeth can be expt’lly induced

  • Gastrotheca guentheri – re-evolved true teeth


Slide15 l.jpg


Example of phenetics gone wrong l.jpg
Example of phenetics gone wrong classification that may not show evolutionary history

  • Limpet, Barnacle, Lobster:

  • But, lobster & barnacle more closely related…convergence

Barnacle

Limpet

Lobster


Cladistics l.jpg
Cladistics classification that may not show evolutionary history

  • Greek: klados = branch

  • Join spp. into truly MONOPHYLETIC groups (avoid pitfalls of phyletic approach)

  • Hennig (1979) - key to monophyletic groups:

    Unique Synapomorphies: shared, derived characters

  • Focus on CLADOGENESIS, ignores anagenesis


Principles of cladistics l.jpg
Principles of Cladistics classification that may not show evolutionary history

  • All spp. in group share common ancestor

  • Include all descendants

  • Bifurcate branching:

    No reticulation

    - Joining of separate lineages on a phylogenetic tree via hybridization or lateral gene transfer


Ancestral traits l.jpg
Ancestral traits classification that may not show evolutionary history

Criteria to determine primitiveness:

  • Presence in fossils

  • Commonness across taxa

  • Early appearance in phylogeny

  • Presence in outgroup


Cladograms l.jpg
Cladograms classification that may not show evolutionary history

1) select group of organisms

2) determine characters & states

3) for each character, classify ancestral & derived

- comparison to outgroup

- traits shared with outgroup = ancestral

4) group by shared derived characters (synapomorphies)

5) choose most parsimonious tree

(fewest evolutionary transitions)


Example seed plants l.jpg
Example: Seed Plants classification that may not show evolutionary history


Parsimonious tree l.jpg
Parsimonious Tree classification that may not show evolutionary history


Complications l.jpg
Complications classification that may not show evolutionary history

  • When only differ in 2 aspects: how decide what is most ancestral?

    1) Complexity…

  • e.g. Bipedalism

    & Internal Dev’t

  • Bipedalism more

    likely to evolve 2 x

    than int. dev’t


2 sines lines l.jpg
2) SINEs & LINEs classification that may not show evolutionary history

  • Short & Long Interspersed Elements

  • Parasitic DNA sequences

  • Can use as phylogenetic characters

  • Insertion rare, unlikely to get same insertions from different events

  • Reversal detectable because lose part of host genome too

  • Homoplasy unlikely,  reliable characters

  • Helped to determine place of whales in artiodactyla