Estimating divergence and migration times of sticklebacks (Gasterosteidae): a fossil-based analysis using the Adaptive Beta method

The timing of major events in stickleback evolution is important for determining rates of molecular evolution within the group. Inferred substitution rates depend on fossil-calibrated divergence times, but in sticklebacks these currently rest on the single oldest fossil occurrence or calibrations largely from other fish taxa, leaving them vulnerable to the idiosyncrasies of the fossil record. We infer a chronology for major events in stickleback evolution using the Adaptive Beta method (ABM) on subsets of a record of 53 gasterosteid fossil occurrences. By incorporating multiple fossil ages in a Bayesian framework, this approach produces credible intervals that account for varying fossil recovery potential through time. Our analysis indicates that the Gasterosteidae originated approximately 32.9 Mya (95% CI: 24.5-54.3 Mya), with the divergence between Gasterosteus and other genera, including Pungitius, occurring at approximately 20.8 Mya (95% CI: 13.8-40.7 Mya). Furthermore, the results inform the timing of inter-oceanic dispersal: we find that Gasterosteus aculeatus Linnaeus, 1758 likely entered the Atlantic only 0.9 Mya (95% CI: 0.6-1.7 Mya). Inclusion of uncertain early Atlantic basin fossils pushes this invasion back to 6.4 Mya (95% CI: 4.8-10.4 Mya). These interval-based estimates provide a reliable chronological framework for calibrating rates of nucleotide substitution in sticklebacks.