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How do we infer phylogeny?

How do we infer phylogeny?. 3 “schools” of phylogenetic thought: Evolutionary systematics Phenetics Cladistics/phylogenetics. 1. Evolutionary systematics. -Arose during the Modern Synthesis of Evolution (Ernst Mayr, Theodosius Dobzhansky, G.G. Simpson)

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How do we infer phylogeny?

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  1. How do we infer phylogeny? • 3 “schools” of phylogenetic thought: • Evolutionary systematics • Phenetics • Cladistics/phylogenetics

  2. 1. Evolutionary systematics -Arose during the Modern Synthesis of Evolution (Ernst Mayr, Theodosius Dobzhansky, G.G. Simpson) -Tried to be synonymous with evolutionary biology & “Neo-Darwinism” -Goal: Think of relationships among organisms as how Natural Selection made them. -Very little (if any) methodology or “operationalism” Construct scenarios, but no formal system of theories. -Difficult to formulate testable hypotheses.

  3. 1. Evolutionary systematics -Often only classifications, with little attempt to depict relationships as “trees” (phylogenies). -”Trust the experts”

  4. 2. Phenetics -Emphasizes the overall similarity of PHENOtypes in grouping and classifying taxa. -Maintains principles of Neo-Darwinism, but NO ESTIMATION OF PROCESSES. -Largely methodological/operational. NO PHILOSOPHICAL BASIS. -Uses any and all data, as long as it can be quantified. -Resulting “trees” called “Phenograms.” Statements of SIMILARITY ONLY. Useful for summarizing resemblence

  5. 2. Phenetics: “phenograms”

  6. 3. Cladistics/phylogenetics (Hennig) -Founded on principles of Operational Darwinism 1. Darwinian Evolution= “Descent with modification” 2. Phylogeny is the result of evolution 3. Therefore, focus on derived MODIFICATIONS for evidence of phylogeny. -Cladistics uses ONLY shared,derived features to infer phylogeny (Evolutionary Systematics & Phenetics use ALL features). -Need to distinguish ANCESTRAL vs. DERIVED

  7. Terms & concepts used in phylogenetics/cladistics CHARACTER: Heritable trait possessed by an organism; characters are usually described in terms of their states, for example: "hair present" vs. "hair absent," where "hair" is the character, and "present" and "absent" are its states.

  8. Terms & concepts used in phylogenetics/cladistics HOMOLOGY: Characters are considered homologous when they are inherited from a common ancestor which possessed that feature. HOMOPLASY: A similar feature shared by two or more taxa that does not meet the criterion (or criteria) of homology. Homoplasies generally arise via convergence. CONVERGENCE: the independent (convergent) evolution of anatomical or functional similarity between unrelated or distantly related lineages or forms. The resulting similarities are only superficial, generally resulting from similar adaptation to similar environments and are NOT a result of common ancestry (and are therefore NOT homologies).

  9. humerus Bat humerus Bird humerus Pterosaur

  10. Lizards & snakes Crocodiles Dinosaurs & birds Mammals & reptile-like mammals Turtles Anapsida Diapsida Amphibians Saurapsida Synapsida -ca. 320 mya most recent common ancestor Amniota -evolution of cleidoic (shelled) egg; ca. 350 mya

  11. Terms & concepts used in phylogenetics/cladistics APOMORPHY: a derived feature or character; derived from and differing from an ancestral (plesiomorphic) condition. SYNAPOMORPHY: A shared, derived character (apomorphy) reflecting common ancestry used to group taxa. Hair is a synapomorphy of mammals.

  12. Terms & concepts used in phylogenetics/cladistics PLESIOMORPHY: An ancestral or primitive character, often incorrectly used to group taxa. SYMPLESIOMORPHY: A plesiomorphy shared by two or more taxa.

  13. REMEMBER CHARACTER STATES are primitive or derived. ORGANISMS are not!

  14. How do we identify “apomorphic” vs. “plesiomorphic”? • Fossil record • Ontogeny/embryology e.g., clavicles in deer • Outgroup comparison

  15. Parsimony Criterion Parsimony: The “rule of simplicity.” Simply stated, according to the principle of Maximum Parsimony, accept the explanation requiring the fewest assumptions. Parsimony is the fundamental assumption of traditional cladistics/phylogenetics.

  16. Parsimony Criterion Parsimony: The “rule of simplicity.” Simply stated, according to the principle of Maximum Parsimony, accept the explanation requiring the fewest assumptions. Parsimony is the fundamental assumption of traditional cladistics/phylogenetics. Other criteria: Maximum likelihood; probabilistic criteria (e.g., Bayesian posterior probabilities).

  17. Parsimony Criterion In phylogenetics, we use the parsimony criterion to “optimize” (=minimize) the number of transitions (=steps) from one character state to another, for all characters, on every possible tree, and select the tree or trees that require the fewest number of steps (ad hoc hypotheses).

  18. How many possible trees?

  19. Example

  20. “Nothing in biology makes sense except in the light of evolution.” T. Dobzhansky “Nothing in evolution makes sense except in the light of phylogeny.” Society of Systematic Biologists

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