Lecture 10: Evolution &amp; Classification cont’d

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# Lecture 10: Evolution Classification cont d - PowerPoint PPT Presentation

Lecture 10: Evolution &amp; 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

Species Classification:

• Phenetic: physical attributes, numerical taxonomy
• Evolutionary: synthesis of the two

Reflect Philosophical Differences

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

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 =

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

Character Matrix

S-value Matrix

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

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

Mosaic Evolution
• ancestral & derived characters differ among lineages
• different characters evolve at different rates
Why retained?
• Developmental Canalization
• Character change requires change

in developmental program (rare)

• Useful in large number of ecological contexts

e.g. Rodentia - incisors conserved

- legs evolved rapidly

Homoplasy

 # characters used,  chance of homoplasy

• Convergent Evolution
• Similar phenotypic response to similar ecological conditions
• Different developmental pathways
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

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?
• 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
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
• Limpet, Barnacle, Lobster:
• But, lobster & barnacle more closely related…convergence

Barnacle

Limpet

Lobster

• 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
• 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

Criteria to determine primitiveness:

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

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
• 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
• 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