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Lecture 10: Evolution & Classification cont’d Species Classification: Phenetic: physical attributes, numerical taxonomy Cladistic (Phylogenetic): evolutionary.

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Presentation on theme: "Lecture 10: Evolution & Classification cont’d Species Classification: Phenetic: physical attributes, numerical taxonomy Cladistic (Phylogenetic): evolutionary."— Presentation transcript:

1 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

2 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)

3 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 =

4 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…..

5 Example Taxon12345678910 A0110001110 B0001110111 C0010010001 D1100011110 Character Matrix TaxonABCD A-- B--0.50.4 C--0.3 D-- S-value Matrix

6 Joining Clusters Closest: A&D = 0.7 2 nd 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

7 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

8 Unfortunately… 1.Mosaic Evolution: differences in rate of change of characters in a lineage 2. Homoplasy: shared characters not in common ancestor (analogy)

9 Mosaic Evolution ancestral & derived characters differ among lineages different characters evolve at different rates HumansFrogs # aortic arches Derived (only 1 left) Retained (2) # digitsRetained (5) Derived (4 –front)

10 Why retained? A)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

11 Homoplasy  # characters used,  chance of homoplasy A.Convergent Evolution Similar phenotypic response to similar ecological conditions Different developmental pathways

12 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

13 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

14 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

15 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

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

17 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

18 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

19 Ancestral traits Criteria to determine primitiveness: Presence in fossils Commonness across taxa Early appearance in phylogeny Presence in outgroup

20 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)

21 Example: Seed Plants TAXACotyledon # CarpelsPerianthSeeds Conifers2--Present Dicots2Present Gnetales2-Present Monocots1Present present Outgroup2---

22 Parsimonious Tree

23 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

24 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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