<b>REPEATED MARINE-TO-FRESHWATER FISH TRANSITIONS REVEAL PALEOENVIRONMENTAL MODULATION OF ADAPTIVE RADIATION</b>

Although ecological opportunity in Sahul rivers enabled repeated colonization in the absence of dominant primary freshwater incumbents, younger freshwater lineages nevertheless diversify faster than older ones, contradicting the expectation that early arrivers should undergo elevated diversification when accessing vacant niche space. Although some colonizations coincide with bursts of speciation consistent with adaptive radiation, many yielded few species despite long residence times.

Functional trait analyses likewise revealed no consistent relationship between colonization timing, ecological breadth, or lineage diversification rate, although expanded functional space characterizes previously proposed Sahul adaptive radiations. Comparisons with paleoenvironmental curves indicate that colonization success correlates with sea-level minima and low-oxygen conditions, suggesting that Earth history dynamics modulated when ecological opportunity was accessible.

Our results show that although ecological opportunity enabled repeated freshwater invasions into the Sahul region, diversification outcomes are governed by the interaction of paleoenvironmental dynamics and possibly lineage-specific traits, generating stark asymmetries in freshwater radiations.</p><p><br></p>