Repeated marine-to-freshwater fish transitions reveal paleoenvironmental modulation of adaptive radiation.

Marine-to-freshwater fish transitions in Sahul reveal that paleoenvironmental factors, not just ecological opportunity, shape adaptive radiation outcomes, with 27-34 transitions during the Cenozoic.

• Why it matters: Understanding why some marine lineages diversify extensively while others remain species-poor is crucial for unraveling the drivers of vertebrate evolution and adaptive radiation in freshwater systems.
• What they did: A densely sampled, time-calibrated phylogenomic analysis of 2,303 teleost species identified multiple marine-to-freshwater transitions, examining their timing, ecological traits, and diversification patterns in relation to paleoenvironmental data.
• The result: Diversification success correlates with sea-level lows and low-oxygen conditions, indicating that paleoenvironmental dynamics, rather than solely ecological opportunity, modulate freshwater radiations and their asymmetries.