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Bacon et al.(1)“reject a 3.5 Ma assumption” for the closure of the Central American Seaway, a conclusion that echoes earlier studies that established that the “bulk [of marine ‘geminate’or ‘transisthmian’species on opposite sides of the Isthmus] were split at some point during the long period of geological upheavals associated with the rising Isthmus,”∼ 15–2.8 Ma (2). However, Bacon et al.(1) also argue that large DNA sequence divergences between some transisthmian lineages reveal that the Isthmus geographically isolated marine species as early as 24 Ma. Although Bacon et al.(1) consider the impact of random error on their results, they ignore several well-known sources of error that will systematically bias their results toward overrepresentation of older divergences that were not Isthmus-driven. The critical assumption used by Bacon et al., that the temporal distribution of divergence times across the Isthmus …