BIS 2C Lecture Notes - Lecture 2: Chronogram, Polyphyly, Synapomorphy
17 Jan 2018
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Bifurcating trees - each internal node gives rise to 2 descendant branches. Polytomy - a node in which 3 or more branches descend and they generally reflect uncertainty about phylogenetic relationships. There are three possible resolutions (aka three possible bifurcating trees) for each three way polytomy. The number of possible bifurcating tree solutions goes up extremely quickly as the node increases in polytomy. Clade (aka monophyletic group) - contains a common ancestor and all descendants. For every node in a tree you effectively define a monophyletic group. If a tree has n nodes (internal nodes), then there are n monophyletic groups in that tree (excluding terminal taxa) Non-monophyletic groups - include some but not all descendants of an ancestor. Paraphyletic group - it contains the most recent common ancestor but not all descendants. Ex: reptiles (in the old sense of the word, excluding birds) In the new sense of the word reptiles, we do include birds in the clade.
