Phylodynamics researchers hit a fundamental wall — what genetic sequences alone can never reveal

A new mathematical framework shows that inferring the hidden genealogy of a population from genetic sequence data has hard limits that no amount of additional data can overcome. The framework decomposes these limits into learnable versus unknowable components, meaning some questions about population history can be answered from sequence data, but others are structurally unanswerable without additional information like historical records or fossil dates.

Phylodynamics is used to infer outbreak histories, understand disease spread, and reconstruct evolutionary relationships — all from genetic sequences alone. This paper proves that even with perfect data and unlimited samples, certain aspects of population genealogy remain invisible to sequence data. This matters because researchers and public health agencies routinely use phylodynamic inference to make claims about when transmission happened or how populations split, without knowing which claims rest on solid ground and which are artifacts of the method. The framework gives researchers a practical way to audit which of their inferences are actually supported by sequence data versus which require external assumptions.