Lecture 10: Evolution & Classification cont’d

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 Character Matrix S-value Matrix

6. 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

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… • 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

10. 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

11. Homoplasy  # characters used,  chance of homoplasy • 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

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