Figure 1

Different ways that fossil and molecular data date lineages. Time intervals defined by the horizontal dashed lines and vertical arrows pertain to age estimates for the divergence between hypothetical lineages X and Y. Even with a complete fossil record and perfect molecular clock a discrepancy is expected between fossil (FA) and molecular (MA) age estimates. As diagnostic morphological characters generally evolve (T _Morphology) after species divergence (T _Species), the fossil record will always underestimate (by δ _{Diagnostic character} ) the true speciation time. Genetic data, on the other hand, will overestimate speciation time (by δ _Coalescence ), as polymorphisms present during species divergence will coalesce some time in the past (T _Gene; related to the ancestral species effective population size). The genuine difference between molecular and morphological divergence times will thus be δ _{True MA-FA} . With a less complete fossil record, the oldest known fossil is unlikely to temporally correspond precisely to the origination of a diagnostic character delimiting X and Y, further decreasing FA by δ _{Oldest fossil} . Under the more realistic scenario of lineage-specific rate heterogeneity and limited taxon/character sampling, errors associated with molecular methods (δ _{Clock error} ) may result in overestimation or underestimation of the true speciation time, although underestimates are bounded by the fossil constraint (δ _{Fossil error} ). The observed discrepancy in age estimates, δ _{Realized MA-FA} , may be considerably larger than expectations (δ _{True MA-FA} ).