Lesson 23: Systematics

Phylogenetic Systematics

The field dedicated to the reconstruction and study of evolutionary relationships.

Most Parsimonious

tree with the fewest evolutionary events

Systematists

Scientists who construct evolutionary trees to represent hypotheses about the relationships among species.

Phylogeny

An evolutionary tree depicting relationships among species.

Cladogram

Another term for phylogeny, representing branching evolutionary trees.

Outgroup

A species outside the group being studied, used to determine ancestral character states.

In-group

The group of organisms or taxa being studied in a phylogenetic analysis.

Ancestral State

A character state that arose prior to the common ancestor of the in-group.

Derived State

A character state that is inherited from the most recent common ancestor.

Clade

A group of species that share a common ancestor and include all its descendants.

Synapomorphy

A shared derived character state that defines a clade.

Node

A point of divergence in the tree, representing a common ancestor.

Principle of Parsimony

the phylogeny requiring the fewest evolutionary events is the best hypothesis of relationships.

Homoplasy

A shared character state that is not inherited from a common ancestor, caused by convergent evolution and evolutionary reversal

Convergent Evolution

The process whereby similar traits evolve independently in different lineages.

Evolutionary Reversal

A situation where a trait reverts back to an earlier state.

Molecular Clock

Estimates the timeframe of species divergence events based on DNA mutation rates.

Monophyletic Group

A group that includes the most recent common ancestor and all its descendants.

Paraphyletic Group

A group that includes the most recent common ancestor but not all its descendants.

Polyphyletic Group

A group that does not contain the most recent common ancestor of all members.

Comparative Biology

The application of phylogenetics to illustrate patterns of evolution and species diversification.

HIV/SIV Evolution

The study showing HIV evolved multiple times from SIV when transmitted from primates to humans.

Disease Evolution and Transmission

Using phylogenetics to trace how diseases like HIV spread and to identify sources of infection.

Cladistics

A method of classifying organisms based on shared derived characteristics to determine evolutionary relationships.

Statistical approaches

methods that use probability models to estimate evolutionary trees