C.S. Pierce, a giant in the field of semiotic, splits signs into three categories—Icons, Symbols, and Indexes—based on how they connect to what they represent, with:
- Icons: Signs that represent by “likeness” or “resemblance” (he uses a portrait as an example).
- Indexes: Signs that represent by “physical connection” or “causal relation” (like smoke for fire).
- Symbols: Signs that represent by “convention” or “habit” (like words in a language).
According to this typology of signs, some of your images represent genuine symbols, others icons, whereas for some, it seems more difficult to decide whether they are symbols or icons. But the interesting point here is that tree rings are unambiguously indexes, not symbols, whereas codons lean strongly toward symbols.