Worked example 1
Three species of flightless birds, the Ostrich (Africa), the Rhea (South America), and the Emu (Australia), live on different continents but share similar morphological features (long legs, long neck, large body). Molecular analysis of the protein cytochrome c was performed. The Rhea was found to have 2 amino acid differences compared to the Ostrich, while the Emu had 6 amino acid differences compared to the Ostrich. How does this molecular evidence refine our understanding of their classification?
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This problem demonstrates how molecular data can override conclusions based on morphology.
- Initial Hypothesis (Morphology): Based on their similar body plans, one might assume the Ostrich, Rhea, and Emu are all very closely related. This similarity is likely due to convergent evolution as they adapted to similar grassland environments.
- Analysis of Molecular Data: Molecular evidence, like amino acid sequences, provides a more direct measure of genetic and evolutionary relatedness. Fewer differences in the sequence imply a more recent common ancestor.
- Interpretation:
- Ostrich vs. Rhea: 2 amino acid differences.
- Ostrich vs. Emu: 6 amino acid differences.
- Conclusion: The smaller number of differences between the Ostrich and the Rhea indicates they are more closely related to each other than either is to the Emu. The molecular data suggests that the Ostrich and Rhea share a more recent common ancestor. The Emu, despite its similar appearance, is more distantly related. This refines the classification by showing that the group of large, flightless birds is not as uniform as morphology suggests.