BIS2C_Lect3_Ward_Building_trees_ppt_for_printing

BIS2C_Lect3_Ward_Building_trees_ppt_for_printing - BIS2C...

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1 BIS2C Winter 2012 (Ward) Lecture 3. Building trees Today’s lecture (11 Jan) Film: Discovering the Great Tree of Life Building trees with parsimony Friday’s lecture Building trees (cont d) Model-based approaches Interpreting trees Some organisms are more closely related to one another than to others, i.e., they share a more recent common ancestor . How do we estimate phylogenies? Phylogenetic trees are inferred from the similarities that are observed among species or populations (terminal taxa). A tree is an hypothesis that can be tested with additional data. Traits or features of organisms are referred to as characters . To infer phylogenies we need characters that exhibit some variation, i e characters that show different character state i.e., characters that show different character states . Examples of characters with discrete character states: Character Character states 1. Wings present, absent 2. Eye color red, white, brown 3. Leg length short, long 4. No. of antennal segments 11, 12 5. Nucleotide site (DNA) adenine, thymine, guanine, cytosine How do we estimate phylogenies? For ease of data management we can apply codes to the character states. Character Character states Codes 1. Wings present, absent 0, 1 2. Eye color red, white, brown 0, 1, 2 3. Leg length short, long 0, 1 4. No. of antennal segments 11, 12 0, 1 5. Nucleotide site (DNA) adenine, thymine… A, T, G, C How do we estimate phylogenies? Next we need to record the character states for each of the terminal taxa that we are studying. For this purpose we prepare a data matrix (taxon-by-character matrix). Characters 1234567891 0… morphology DNA sequences Taxa B a t 101000ACCT R a t 011001GCCT C a t 010112GTCT D o g 010112GTTT Characters B a t R a t C a t Dog 0 1 0 1 1 2 G T T T Now we need to evaluate the similarity between taxa and create a tree. T…
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2 Characters 1234567891 0… Taxa B a t 101000ACCT R a t 011001GCCT C a t 010112GTCT If we apply the principle of parsimony then we search for a tree that requires the least amount of change in character states (the fewest steps
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BIS2C_Lect3_Ward_Building_trees_ppt_for_printing - BIS2C...

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