Phylogeny and Shared Derived Traits

Key Concepts

• Clade: A group of organisms that includes all descendants of a common ancestor.
• Synapomorphy: A shared derived trait that is used as evidence to infer clades.

Examples of Synapomorphies

• Feathers: A synapomorphy of all birds (e.g., ostrich, eagle, crow).
• Amnion: A synapomorphy of amniotes (reptiles, mammals, birds).
• Tetrapod Limb: A synapomorphy for tetrapods (amphibians, reptiles, mammals, birds).
  • Note: Traits can be lost over time (e.g., snakes losing limbs).

Importance of Inferring Phylogeny with Shared Derived Characters

• Use shared derived characters instead of primitive characters to determine clades.
• Primitive characters can lead to erroneous conclusions.

Example Analysis of Bird and Mammal Clades

Synapomorphies in Birds

• Feathers
• Right Aortic Arch
• Air-Filled Bones
• Lungs with Air Sacs

Synapomorphies in Mammals

• Hair
• Left Aortic Arch
• Mammary Glands
• Non-Nucleated Red Blood Cells
• Three Middle Ear Bones

Using Outgroups

• Outgroups help determine primitive states.
• Example Outgroup: Turtle
  • Traits: Oviparous, cloaca, interclavicle and coracoid bones, nucleated red blood cells, single middle ear bone, double aortic arch.

Mammal and Bird Trait Analysis

• Mammals: Hair, mammary glands, non-nucleated RBCs, three middle ear bones.
• Birds: Feathers (modification of scales), right aortic arch, lungs with air sacs.
• Placental Mammals: Separate openings for systems, viviparous.

Primitive Traits vs. Derived Traits

• Primitive traits: Oviparity, cloaca, interclavicle, coracoid bone, etc.
• Derived traits are new and arise in specific clades.

Homoplasy

• Traits like live birth arose independently (e.g., in rattlesnakes, mammals, sharks).

Conclusion

• Evidence for clades must be based on shared derived character states through outgroup analysis.
• Avoid using primitive character states alone as they can mislead phylogenetic analysis.