Thanks for adding this to my conceptual library! I like the idea.
One thing I feel uncertain about (not coming from a background in evo bio) is how much evidence the “only evolved once” thing is for contingency. My naive guess would be that there’s an asymmetry here, i.e., that “evolved many times” is a lot of evidence for convergence, but “evolved only once” is only a small amount of evidence for contingency (on its own). For instance, I can imagine (although I don’t know if this has happened) that something highly favorable might evolve (such as a vascular system) and then spreads so quickly that it subsumes the “market,” leaving no room for competitors? In this world, the feature might still be convergent in the sense that if you reran the tape of life you’d find it every time, even though you only saw it evolve once in our tape. I imagine there are other factors here which matter, too. Does that seem wrong?
Also, as an aside, my understanding is that the genetic code is not as contingent as it first seems. For instance, codons “semantically” near each other, e.g., with overlapping bases, also code for chemically similar amino acids. Similarly, the level of robustness the genetic code has (to translation errors) is extremely unlikely, i.e., if one were to scramble the code, you’d only get that robustness in something like one out of 10^38 possible codes (can’t remember the exact number right now, sorry, but it’s astronomically large).
I agree that “only evolved once” (and then “subsumed the market”) is not that much evidence for contingency on its own. But if you combine it with the knowledge that its evolution was facilitated/enabled by some random-ish, contingent factors, then the contingency case is much stronger. For example, the great diversification of mammals was enabled by the extinction of dinosaurs, itself caused by an asteroid impact, a factor as contingent as you can ask.
Thanks for adding this to my conceptual library! I like the idea.
One thing I feel uncertain about (not coming from a background in evo bio) is how much evidence the “only evolved once” thing is for contingency. My naive guess would be that there’s an asymmetry here, i.e., that “evolved many times” is a lot of evidence for convergence, but “evolved only once” is only a small amount of evidence for contingency (on its own). For instance, I can imagine (although I don’t know if this has happened) that something highly favorable might evolve (such as a vascular system) and then spreads so quickly that it subsumes the “market,” leaving no room for competitors? In this world, the feature might still be convergent in the sense that if you reran the tape of life you’d find it every time, even though you only saw it evolve once in our tape. I imagine there are other factors here which matter, too. Does that seem wrong?
Also, as an aside, my understanding is that the genetic code is not as contingent as it first seems. For instance, codons “semantically” near each other, e.g., with overlapping bases, also code for chemically similar amino acids. Similarly, the level of robustness the genetic code has (to translation errors) is extremely unlikely, i.e., if one were to scramble the code, you’d only get that robustness in something like one out of 10^38 possible codes (can’t remember the exact number right now, sorry, but it’s astronomically large).
I agree that “only evolved once” (and then “subsumed the market”) is not that much evidence for contingency on its own. But if you combine it with the knowledge that its evolution was facilitated/enabled by some random-ish, contingent factors, then the contingency case is much stronger. For example, the great diversification of mammals was enabled by the extinction of dinosaurs, itself caused by an asteroid impact, a factor as contingent as you can ask.