Phylogenetic Trees

Phylogenetic Trees

• Phylogeny: evolutionary history of a species or group
• vs. Taxonomy: groups organisms based on similarities
• Show evolutionary relationships between taxa
• Represent hypotheses about evolutionary history
• Build trees using:
• Morphology
• Behavior
• DNA
• Biochemistry

• Taxon (Taxa): the taxonomic unit at any level Example Animalia is the Kingdom taxon
• Sister taxa: Two or more taxa next to each other on the tree (i.e. closely related) Example: Taxon B and Taxon C are sister taxa
• Clade: A group of taxa that share a common ancestor Example: Taxon A, B, C form a clade
• In group: a group of interest usually the focus of a study
• Out group: one or more taxa that is distantly related to the In group
• Character: a specific characteristic Example: a wing or a tail
• State: a particular property of a character Example: the shape of the wing, or the color of the tail

Phylogenetic Trees and Parsimony

• The best hypothesis is the one with the least number of assumptions
• A good phylogeny minimizes the number of assumed character changes

• Synapomorphy: a shared, derived trait that unites a clade Example: Having four legs is a synapomorphy that unites Frogs, Turtles and Leopards
• Symplesiomorphy: a shared, ancestral trait Example: Within the clade of Frogs, Turtles and Leopards, having four legs is a symplesiomorphy

• Monophyletic groups (clade): group of organisms containing a common ancestor, all of its descendants and nothing else.
• Paraphyletic groups: do not include all descendants of the common ancestor (i.e. missing some taxa)
• Polyphyletic groups: have members whose ancestor is outside of the group (i.e. too many taxa)

Similar Traits

• Analogous traits: appear similar but actually evolved independently
• Convergent evolution: natural selection produces a similar trait from different starting points Example: bat and bird wings
• Homologous traits: similar due to true shared ancestry
• Descended from the same trait in a common ancestor Example: dolphin and whale flippers