Reading Phylogenetic Trees: Basal Taxon and Sister Taxa

Reading a Phylogenetic Tree

• A basal taxon is the oldest and most primitive, representing the earliest lineage that evolved from the root and remains unbranched.
• Sister taxa are two lineages that stem from the same branch point, sharing a common ancestor.
• A polytomy is an unresolved pattern of divergence, where multiple branches emerge from a single point.
• Branch points represent the common ancestor of two or more species.

Key Concepts

• Apomorphic features are derived from an ancestral state, representing evolutionary change.
• An autapomorphy is a unique feature of a single taxon.
• Synapomorphies are shared characteristics among multiple taxa, indicating evolutionary relationships.
• Polytomous taxa form unresolved patterns of divergence.

Phylogenetic Trees

• Represent a hypothesis about evolutionary relationships.
• Each branch point represents the divergence of two species.
• Tree branches can be rotated around a branch point without changing the evolutionary relationships.
• A rooted tree includes a branch to represent the last common ancestor of all taxa in the tree.
• A basal taxon diverges early in the history of a group and originates near the common ancestor group.

Primary Tenet of Phylogenetic Systematics

• All forms of life share a common ancestor.
• Taxa that share a common ancestor more recent in time are more closely related to one another than they are to a taxon whose common ancestor is further back in time.

Molecular Homologies

• DNA and RNA sequences of nucleic acids provide evidence for evolutionary relationships.
• Each change in a nucleic acid represents one evolutionary event.
• More events indicate a more distant relationship, while fewer events indicate a closer relationship.

Cladograms and Clades

• A cladogram is a type of phylogenetic tree.
• A clade is a group of species that includes a common ancestor and all its descendants.
• Monophyletic clades consist of a common ancestor and all its descendants, meeting the cladistic criterion.
• Paraphyletic clades consist of an ancestor and some but not all of the descendants, failing the cladistic criterion.
• Polyphyletic clades consist of various species with different ancestors, lacking a common ancestor.

Constructing Cladograms

• Identify homologies, or shared characteristics derived from one ancestor.
• Select species for the ingroup and outgroup.
• Select an outgroup with no shared derived characters but shared primitive characters.
• Construct a character table and tabulate the data.
• Construct a cladogram based on the number of shared characters.