CLADISTICS 2

CLADISTICS 2

1. PHENETICS

   • hierarchical clustering of organisms according to their morphological and/or molecular similarities
   • for any given 3 organisms - 3 distinct reasons why 2 might share a feature but not the 3rd
     • CONVERGENCE
       • similarity resulting from independent adaptation to similar conditions eg similar body shape of whales & sharks compared to land-dwelling mammals - explained by common adaptation to reduce drag in water
     • feature in question in common ancestor of all 3 but since lost in 1 eg lizards & crocs have legs but snakes don't
     • 2 organisms inherited feature from common ancestor where it originated but they were already separate from evolutionary line leading to 3rd eg suckling young in deer & whale but not shark

2. cladistics different to phonetics as aim is to infer relationships exclusively from 3rd kind of similarity

   • shared derived states of characters - to cluster organisms in way that directly reflects phylogeny
     • challenge - distinguish similarities of this kind from other 2 kinds

3. neither cladistics or phenetics entirely free of element of subjectivity

4. 2 main sources of evidence of evolutionary relationships

   • bodily attributes - morphology, behaviour etc
   • genes - DNA sequences
   • able to trace back ancestral relationships by looking at where base attributes have changed

5. dichotomous branching

   • 3 branches for 1 junction - conventional

6. deciding whether 2 features homologous difficult - organisms may acquire similarities through convergent evolution from unrelated ancestors

   • HOMOPLASY
     • corroborated by repetition on unconnected branches in most parsimonious cladograms
   • fish pectoral fins share similar shape & function as birds wings but not same structure eg no bones
     • as vertebrate appendage - homologous
     • as fin - dolphin is analogous to that of pectoral fin in fish

7. in technical terms, novel or 'derived' states of character said to be apomorphic relative to primitive or plesiomorphic state

8. SYNAPOMORPHY

   • shared apomorphies - inherited from common ancestor where novelty arose
   • serve to order relative recency of common ancestry among species & branching nodes in phylogenetics

9. APOMORPHY

   • derived character state - transformed from different primitive character state in ancestor

10. PLESIOMORPHY

    • character state designated primitive condition within group of taxa