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Cladistic Systematics

Cladistic Systematics. Now that we know how to read phylogenetic trees….how are they made? Based on shared anatomical characteristics Cladogram : a branching diagram that shows the relationship among species in regard to their shared derived characteristics

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Cladistic Systematics

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  1. Classification of Living Things 20-3

  2. Cladistic Systematics • Now that we know how to read phylogenetic trees….how are they made? • Based on shared anatomical characteristics • Cladogram:a branching diagram that shows the relationship among species in regard to their shared derived characteristics A cladogram is a special type of phylogenetic tree

  3. Cladistic Systematics • Cladogram: Traces evolutionary history of the group under study • Uses shared derived characters to: • Classify organisms, and • Arrange taxa into a cladogram • A clade is an evolutionary branch that includes: • A common ancestor, together with • All its descendent species

  4. Constructing a Cladogram When constructing a cladogram, the first step is to make a chart with the characteristics to be compared. Notice in the chart on the right all of the organisms have a notochord, but not all of them are vertebrates.

  5. Constructing a Cladogram Only the newt, snake and lizard have lungs and a 3-chambered heart. Snakes and lizards have internalization fertilization and amniotic membranes in their eggs. Reptiles (snakes and lizards) are a clade Clade: an evolutionary branch that includes a common ancestor together with all its descendant species

  6. Constructing a Cladogram The newt (an amphibian) and the eel (fish) are all evolutionary descendants of reptiles The next step is to draw a cladogram showing these relationships The goal is to have the simplest drawing possible What is the deal then with bony limbs and long cylindrical body?

  7. Constructing a Cladogram parsimony: the fewest number of assumptions is the most logical. results in the simplest cladogram possible

  8. Parsimony • Cladists are always guided by the principle of parsimony • The arrangement requiring the fewest assumptions is preferred • This would: • Leave the fewest number of shared derived characters unexplained • Minimize the number of assumed evolutionary changes • The reliability of a cladogram is dependent on the knowledge and skill of the investigator

  9. Alternate, Simplified Cladograms • X, Y and Z share the same characteristics. • b. & c. b. and c. are unlikely examples as the shared characteristics in Y and Z would have had to evolve independently

  10. Cladistic Versus Traditional View ofReptilian Phylogeny

  11. Phenetic Systematics • Assumes it will never be possible to construct a truly phylogenetic classification system • Species are classified according to the total number of shared similarities • Disregards assumed phylogenetic considerations • Ignores issues of convergent or parallel evolution

  12. Traditional Systematics • Mainly uses anatomical data • Classify organisms using assumed phylogeny with emphasis on phenotype • Stress both common ancestry and degree of structural difference among divergent groups • Construct phylogenetic trees by applying evolutionary principles to categories • Not strict in making sure all taxa are monophyletic

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