Refer to the figure showing the hypothetical observed sequences from two related species and their common ancestor. 
If we only had information from species 1 and species 2, how many substitutions could be inferred?
Definitions:
Glossopteris
An extinct genus of seed fern that serves as evidence for the theory of continental drift and once thrived in the supercontinent Gondwana.
Archaeopteryx
An extinct genus of bird-like dinosaurs that is considered by many to be the earliest known bird, illustrating the transition between non-avian dinosaurs and birds.
Homologous Structures
Anatomical features in different species that share a common ancestral origin despite possibly serving different functions.
Comparative Anatomy
The study of similarities and differences in the anatomy of different species to understand their evolutionary relationships.
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