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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
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
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
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
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
Example

Character Matrix

S-value Matrix

joining clusters
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
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
unfortunately
Unfortunately…
  • Mosaic Evolution: differences in rate of change of characters in a lineage

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

mosaic evolution
Mosaic Evolution
  • ancestral & derived characters differ among lineages
  • different characters evolve at different rates
why retained
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
Homoplasy

 # characters used,  chance of homoplasy

  • Convergent Evolution
  • Similar phenotypic response to similar ecological conditions
  • Different developmental pathways
slide12
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
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
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
Homoplasy & variation in rate of character change = phenetic classification that may not show evolutionary history
  • Can get :

monophyletic, paraphyletic, polyphyletic

groups b/c use all character types

example of phenetics gone wrong
Example of phenetics gone wrong
  • Limpet, Barnacle, Lobster:
  • But, lobster & barnacle more closely related…convergence

Barnacle

Limpet

Lobster

cladistics
Cladistics
  • 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
Principles of Cladistics
  • 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
Ancestral traits

Criteria to determine primitiveness:

  • Presence in fossils
  • Commonness across taxa
  • Early appearance in phylogeny
  • Presence in outgroup
cladograms
Cladograms

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)

complications
Complications
  • 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
2) SINEs & LINEs
  • 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
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