Biology subjects

Thacker, C. E.

Publications and source records attributed to Thacker, C. E..

3 recordsLinked to original sources

bioRxiv2026

The macroevolutionary impact of an innovation reversal in ray-finned fishes

The evolution of new traits can drive species diversification by facilitating the use of new resources, but environmental change may turn these same adaptations into liabilities.Trait loss is also often associated with the origin of new ecologies, but how losses modulate diversification remains unclear. The swim bladder allows ray-finned fishes to regulate their buoyancy and exploit ecosystems throughout the water column, yet this organ has been lost many times among species-rich lineages. Here, we show that timing and ecological context control the macroevolutionary effects of swim bladder loss. Many lineages of fishes lost the swim bladder over the last 66 million years as they specialized for benthic habitats where buoyancy regulation is unnecessary. Swim bladder loss enabled the descendants of these benthic fishes to diversify in the deep sea where extreme pressure makes its inflation untenable, and in the frigid, oxygen-saturated Southern Ocean, where loss of the oxygen delivery mechanisms required for swim bladder inflation carries little physiological cost. Yet, we detect a selective filter associated with swim bladder loss during extreme global warming 56 to 50 million years ago, when its absence limited the capacity of fishes to escape ecological disruptions on the ocean floor. These contrasting patterns explain how the loss of a complex trait promoted major ecological transitions without increasing overall diversification through deep time. As human activity drives rapid global warming, the evolutionary legacies of swim bladder loss may again shape the fate of marine fish diversity.

evolutionary biology↗

bioRxiv2026

REPEATED MARINE-TO-FRESHWATER FISH TRANSITIONS REVEAL PALEOENVIRONMENTAL MODULATION OF ADAPTIVE RADIATION

Tropical rivers in Australia and New Guinea (Sahul) provide a rare natural experiment in vertebrate evolution: unlike other continental systems, their freshwater ichthyofaunas are composed almost entirely of marine-derived lineages rather than primary freshwater fishes. This unique biogeographic setting enables replicated tests of why some marine-to-freshwater transitions give rise to extensive adaptive radiations whereas others remain species-poor, and whether these outcomes reflect ecological opportunity or temporally structured paleoenvironmental constraints. Using a densely sampled, time-calibrated phylogenomic framework spanning 2,303 teleost species, we identified a likely range of 27-34 marine-to-freshwater transitions during the Cenozoic, including a pronounced Middle Miocene peak (16-11 Ma). 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 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. Significance statementTropical rivers in Australia and New Guinea host one of the most unusual continental freshwater fish assemblages on Earth, composed almost entirely of marine-derived lineages. This system allows asking why some colonizing lineages diversify dramatically while others remain species-poor on a continental scale. Using large-scale phylogenomic and functional trait data, we show that early arrival alone does not predict diversification success. Instead, the lineages that radiate most successfully are those whose arrival coincides with windows of paleoenvironmental opportunity created by sea-level and oxygen fluctuations. These results reveal that the fates of colonizing lineages are shaped not only by ecological opportunity, but also by Earth-history dynamics that govern when, where, and how species can invade and diversify.

evolutionary biology↗

bioRxiv2021

Prolonged morphological expansion of spiny-rayed fishes following the end-Cretaceous

Spiny-rayed fishes (Acanthomorpha) dominate modern marine habitats and comprise more than a quarter of all living vertebrate species1-3. It is believed that this dominance resulted from explosive lineage and phenotypic diversification coincident with the Cretaceous-Paleogene (K-Pg) mass-extinction event4. It remains unclear, however, if living acanthomorph diversity is the result of a punctuated burst or gradual accumulation of diversity following the K-Pg. We assess these hypotheses with a time-calibrated phylogeny inferred using ultraconserved elements from a sampling of species that represent over 91% of all acanthomorph families, as well as an extensive body shape dataset of extant species. Our results indicate that several million years after the end-Cretaceous, acanthomorphs underwent a prolonged and significant expansion of morphological disparity primarily driven by changes in body elongation, and that acanthomorph lineages containing the bulk of the living species diversity originated throughout the Cenozoic. These acanthomorph lineages radiated into distinct regions of morphospace and retained their iconic phenotypes, including a large group of laterally compressed reef fishes, fast-swimming open-ocean predators, bottom-dwelling flatfishes, seahorses, and pufferfishes. The evolutionary success of spiny-rayed fishes is the culmination of a post K-Pg adaptive radiation in which rates of lineage diversification were decoupled from periods of high phenotypic disparity.

evolutionary biology↗