Basics of Phylogeny – Characters (Ancestral vs. Derived)

Introduction to Phylogeny

Phylogeny refers to the evolutionary history and relationships among different organisms. It is often represented using phylogenetic trees, which depict common ancestors and divergence among species. Scientists use morphological, molecular, and genetic data to reconstruct phylogenies.

Characters in Phylogeny

Characters are heritable traits used to determine evolutionary relationships. These can be morphological (e.g., bone structures), molecular (DNA sequences), physiological, or behavioral traits.

Characters in phylogeny are classified into two main types: Ancestral characters and Derived characters.

1. Ancestral Characters (Plesiomorphy)

• These are primitive traits that were present in a common ancestor and are retained by multiple descendant species.
• They do not provide much information for distinguishing evolutionary relationships between closely related groups.
• Example: Presence of a backbone (vertebral column) in vertebrates is an ancestral character, as it is found in all vertebrates, from fishes to mammals.

2. Derived Characters (Apomorphy)

• These are newly evolved traits that are unique to a particular lineage or group.
• They are useful in determining evolutionary relationships because they distinguish certain groups from their ancestors.
• Example: Feathers in birds are a derived character because their ancestors (dinosaurs) did not have them.

Types of Derived Characters

• Synapomorphy: A shared derived character found in two or more taxa and their most recent common ancestor.
  • Example: Mammary glands in mammals, as they are shared among all mammals but absent in non-mammalian ancestors.
• Autapomorphy: A derived trait unique to a single species or lineage.
  • Example: The ability of humans to speak complex languages is an autapomorphy among primates.

Role of Characters in Phylogenetic Classification

• Identifying Common Ancestry: Helps in tracing the evolutionary history of organisms.
• Grouping Organisms: Species sharing synapomorphies are placed in the same taxonomic group.
• Constructing Cladograms: Derived characters are used in cladistics to build evolutionary trees.