Biology subjects

Sallan, L.

Publications and source records attributed to Sallan, L..

4 recordsLinked to original sources

bioRxiv2026

Phenotypic complexity determines the predictability of molecular convergence

Whether the outcomes of evolution are predictable remains a central question in evolutionary biology. Convergent evolution, in which similar phenotypes arise independently in distinct lineages, is often interpreted as evidence of predictability under shared selective pressures. However, empirical studies have shown that convergent phenotypes can exhibit similar or disparate molecular bases, and no general frame-work predicts when convergence should occur. Here, we show that phenotypic complexity determines the extent of convergence under shared conditions. Using a deliberately minimal abstraction linking genotype and phenotype and evolutionary simulations, we investigated how regulatory gene expression programs evolve under selection for shared phenotypic optima. We find that phenotypes of low complexity can be produced by diverse regulator expression programs, permitting phenotypic convergence despite molecular divergence. However, as the complexity of the target phenotype increases, the variety of viable regulatory solutions decreases, funneling independently evolving lineages towards similar expression programs. In this regime, molecular convergence emerges as a predictable consequence of selection acting on finite regulatory repertoires, consistent with the phenomenon of "deep homology" observed in structures like eyes and limbs. Allowing evolutionary changes in regulator-target interactions relaxes these constraints and enhances divergence in expression between phenotypically convergent species. Together, our results provide general expectations for predicting when phenotypic convergence should be accompanied by convergent genetic mechanisms, and shed light on the causes of convergence in a broad range of phenotypic traits.

evolutionary biology↗

bioRxiv2025

Mass Extinction Triggered the Early Radiations of Jawed Vertebrates and Relatives (Gnathostomes)

Most vertebrate lineages are first recorded from the mid-Paleozoic, well after their Cambrian origin and Ordovician invertebrate biodiversification events. This delay has been poorly understood and is usually attributed to sampling and long ghost lineages. We analyzed new databases of Paleozoic vertebrate occurrences, biogeography, and ecosystems, revealing that the Late Ordovician Mass Extinction (~444-443 million years ago) triggered parallel, endemic radiations of jawed and related jawless vertebrates (gnathostomes) in isolated refugia. Post-extinction ecosystems hosted the first definitive appearances of most major vertebrate lineages of the Paleozoic "Age of Fishes" (and today), following the loss of ubiquitous stem-cyclostome conodonts, nascent faunas of other gnathostomes, and pelagic invertebrates. Turnover and recovery patterns matched those following climatically similar events like the end-Devonian mass extinction, including a post-extinction "gap" with low biodiversity. The prolonged 23 million year Silurian recovery, and the challenges of oceanic dispersal, likely further delayed the dominance of jawed gnathostomes for millions of years after the first fossil jaws.

paleontology↗

bioRxiv2025

A general evolutionary model for the emergence of novel characters from serial homologs

Repeated structures are widespread across multicellular organisms, such as vertebrae, segments, and cell types. These structures, also known as serial homologs, share ancestral states and developmental underpinnings yet also provide the materials for novel adaptive phenotypes. It remains largely unclear why some repeated structures diverge quickly, while others remain constrained. One reason for this uncertainty is the lack of a generalized model for the evolution of repeated structures under different scenarios that links developmental, genetic, and selective constraints to expected and observed patterns of evolution. Here, we introduce a model that incorporates key structural features of gene regulatory networks and selection and investigate how responses to multivariate selection depend on developmental constraints. We show structural features of developmental networks determine when repeats can respond independently to selection and when divergence is limited. Simulations recover broad expectations of phenotypic evolution under directional selection inferred from empirical data. We further show that, in the face of fluctuating selection, strong developmental constraints lead to reduced fitness over time and attenuated fitness fluctuations. Together, our results provide general insights into the principles of evolution of repeated structures and offer a modeling framework for the evolution of a broad range of key phenotypic characters.

evolutionary biology↗

bioRxiv2023

Modification of clinical dental impression methods to obtain dental traits from living and whole non-mammalian vertebrates

Dental impressions, developed for accurate capture of oral characteristics in human clinical settings, are seldom used in research on non-livestock, non-primate, and especially non-mammalian, vertebrates due to a lack of appropriate tools. Studies of dentitions in most vertebrate species usually require euthanasia and specimen dissection, microCT and other scans with size and resolution tradeoffs, and/or ad-hoc individual impressions or removal of single teeth. These approaches prevent in-vivo studies that factor in growth and other chronological changes and separates teeth from the context of the whole mouth. Here, we describe a non-destructive method for obtaining high resolution dentition-related traits that can be used on both living animals and museum specimens for almost all vertebrates, involving a customizable and printable dental impression tray. This method has repeatedly and accurately capture whole-mouth morphology and detailed features at high resolution in the living non-teleost actinopterygian fish, Polypterus senegalus, in a laboratory setting. It can be used for comparative morphology and to observe temporal changes such as the presence of microwear, tooth replacement rates, and occlusal and morphological changes through ontogeny. Summary statementThis study presents a cheap and customizable method using a printable tray to measure oral morphological traits in vertebrates humanely and accurately.

evolutionary biology↗