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Scaffold S17 — Mutation target size and the rate of parallel evolution

Five phenotypes. Each can be produced by some developmental genetic architecture. Some architectures have many parallel paths — many mutations in many genes can yield the same outcome. Others have a single narrow path — only one very specific mutation works. The number of times a trait evolves independently across a clade tracks that "effective mutation rate to the phenotype."

Locked — answer the pretest above first.

Running tally

What you just did has a name

The rate at which a phenotype evolves is not just about selection. It is also about how many mutations can produce it. Webbed feet and eye loss are easy — any of several genes can be tweaked and you get the phenotype. They arise again and again. The placenta is hard — it required a specific transcription factor binding a specific transposon that happened to land in exactly the right place. It evolved roughly twice in the history of vertebrates. Hinged jaws and the tetrapod limb are harder still; those evolved once each.

The pattern in the tally reflects two separate multipliers on the per-generation rate of evolution:

A trait with a huge mutation target that is strongly favored in some environments — eye loss in caves, webbed feet in aquatic lineages — appears in every clade the environment recurs in. A trait with a tiny mutation target — hinged jaws, true flight, bone — appears once or twice in the entire history of life and never again. The parallel-evolution count is a clean empirical readout of this distinction.