Phylogeny and Character Analysis

Introduction

• Examination of relationships among organisms using character states.
• In-group: Elephant, Eagle, Snake.
• Out-group: Salamander (amphibian).

Character States

• Character State: Defines presence, absence, or specific traits (e.g., size, color).

Primitive Characters

• Cells: Present in all living organisms — not useful for determining relationships.
• Vertebrae: All in-group and out-group have vertebrae — primitive condition.

Ectothermy

• Ectothermic: Salamander and Snake.
• Endothermic: Eagle and Elephant.
• Analysis:
  • Ectothermy does not show a close relationship due to its primitive nature.
  • It is inherited from ancestors (e.g., fish), not a derived character.

Amniote Egg

• Found in reptiles, birds, and mammals.
• Indicates that the in-group is a clade but doesn't specify close relationships.

Derived Characters

• Hair: Elephant has hair (unique trait) – an autapomorphy.
• Feathers: Unique to birds (e.g., Eagle) – another autapomorphy.

Diapsid Skull

• Diapsid Skull: Two temporal openings, present in Snakes and Birds (reptiles and birds).
• A synapomorphy indicating a shared ancestor between reptiles and birds.

Endothermy

• Found in both birds and mammals.
• Considered evidence for a relationship but indicates independent development in birds and mammals.
• Classified as a homoplasy (convergent evolution).

Phylogeny Examples

Dorsal Fin Example

• Organisms: Tuna, Lizard, Monkey, Hippopotamus, Whale.
• Dorsal Fin:
  • Present in Tuna and Whale.
  • Considered a homoplasy — developed independently.

Alternative Phylogeny

• Hypothetical phylogeny showing bipedalism in birds and humans as evidence of relation.
• Bipedalism seen as a homoplasy.

Homology vs Homoplasy

• Homology: Shared traits due to common ancestry.
• Homoplasy: Shared traits due to independent evolution.
• Maximal Parsimony: The best phylogenetic tree is the one with the fewest homoplasies.

Conclusion

• In any phylogenetic tree, both homologous and homoplastic characters will be present.
• The most parsimonious tree provides the least number of homoplasies, suggesting the most likely evolutionary path.