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1. Habitat transitions alter the adaptive landscape and shape phenotypic evolution in needlefishes

   https://doi.org/10.1002/ece3.6172  

   Kolmann, M.A. (April 2020, Ecology and evolution)

   Habitat occupancy can have a profound influence on macroevolutionary dynamics, and a switch in major habitat type may alter the evolutionary trajectory of a lineage. In this study, we investigate how evolutionary transitions between marine and freshwater habitats affect macroevolutionary adaptive landscapes, using needlefishes (Belonidae) as a model system. We examined the evolution of body shape and size in marine and freshwater needlefishes and tested for phenotypic change in response to transitions between habitats. Using micro-computed tomographic (μCT) scanning and geometric morphometrics, we quantified body shape, size, and vertebral counts of 31 belonid species. We then examined the pattern and tempo of body shape and size evolution using phylogenetic comparative methods. Our results show that transitions from marine to freshwater habitats have altered the adaptive landscape for needlefishes and expanded morphospace relative to marine taxa. We provide further evidence that freshwater taxa attain reduced sizes either through dwarfism (as inferred from axial skeletal reduction) or through developmental truncation (as inferred from axial skeletal loss). We propose that transitions to freshwater habitats produce morphological novelty in response to novel prey resources and changes in locomotor demands. We find that repeated invasions of different habitats have prompted predictable changes in morphology. 

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2. Scaling patterns of body plans differ among squirrel ecotypes

   https://doi.org/10.7717/peerj.14800  

   Linden, Tate J.; Burtner, Abigail E.; Rickman, Johannah; McFeely, Annika; Santana, Sharlene E.; Law, Chris J. (January 2023, PeerJ)

   Body size is often hypothesized to facilitate or constrain morphological diversity in the cranial, appendicular, and axial skeletons. However, how overall body shape scales with body size ( i.e. , body shape allometry) and whether these scaling patterns differ between ecological groups remains poorly investigated. Here, we test whether and how the relationships between body shape, body size, and limb lengths differ among species with different locomotor specializations, and describe the underlying morphological components that contribute to body shape evolution among squirrel (Sciuridae) ecotypes. We quantified the body size and shape of 87 squirrel species from osteological specimens held at museum collections. Using phylogenetic comparative methods, we first found that body shape and its underlying morphological components scale allometrically with body size, but these allometric patterns differ among squirrel ecotypes: chipmunks and gliding squirrels exhibited more elongate bodies with increasing body sizes whereas ground squirrels exhibited more robust bodies with increasing body size. Second, we found that only ground squirrels exhibit a relationship between forelimb length and body shape, where more elongate species exhibit relatively shorter forelimbs. Third, we found that the relative length of the ribs and elongation or shortening of the thoracic region contributes the most to body shape evolution across squirrels. Overall, our work contributes to the growing understanding of mammalian body shape evolution and how it is influenced by body size and locomotor ecology, in this case from robust subterranean to gracile gliding squirrels. 

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3. Migratory lineages rapidly evolve large body sizes in ray-finned fishes

   https://doi.org/doi.org/10.1098/rspb.2019.2615  

   Burns, M.; Bloom, D.D. (January 2020, Proceedings of the Royal Society of London)

   Migratory animals respond to environmental heterogeneity by predictably moving long distances in their lifetime. Migration has evolved repeatedly in animals, and many adaptations are found across the tree of life that increase migration efficiency. Life-history theory predicts that migratory species should evolve a larger body size than non-migratory species, and some empirical studies have shown this pattern. A recent study analysed the evolution of body size between diadromous and non-diadromous shads, herrings, anchovies and allies, finding that species evolved larger body sizes when adapting to a diadromous lifestyle. It remains unknown whether different fish clades adapt to migration similarly. We used an adaptive landscape framework to explore body size evolution for over 4500 migratory and non-migratory species of ray-finned fishes. By fitting models of macroevolution, we show that migratory species are evolving towards a body size that is larger than non-migratory species. Furthermore, we find that migratory lineages evolve towards their optimal body size more rapidly than non-migratory lineages, indicating body size is a key adaption for migratory fishes. Our results show, for the first time, that the largest vertebrate radiation on the planet exhibited strong evolutionary determinism when adapting to a migratory lifestyle. 

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4. Darter fishes exhibit variable intraspecific head shape allometry and modularity

   https://doi.org/10.1002/ar.25088  

   Goodwin, Maris R.; Arbour, Jessica H. (February 2023, The Anatomical Record)

   Abstract Allometry, the relationship between anatomical proportions and body size, may either limit or facilitate the diversification of morphology. We examined the impact of allometry in darter fish morphology, which displays a variety of trophic morphologies. This study aimed to address (a) whether there was significant variation in darter head allometry, (b) if allometry contributed to head shape diversity in adults, and (c) if darters show head shape modularity associated with allometry. We used geometric morphometrics to quantify head shape across 10 different species and test for heterogeneity in allometric slopes. In addition, we quantified the degree of modularity between the preorbital and postorbital regions of the darter head, both before and after correction for body size. We found that different species have unique allometric slopes, particularly among theSimopercasubgenus, and that closely related darter species tend to show ontogenetic divergence, contributing to the diversity of head shapes observed in adults. We suggest that such a pattern may result from the similarity of juvenile diets due to gape limitation. We also found that several species show significant modularity in head shape but that modularity was evolutionarily labile and only sometimes impacted by head shape allometry. Overall, our work suggests that ontogenetic shape development may have been important to the evolution of head shape in darters, particularly in the evolution of foraging traits and microhabitat. 

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5. Intrahost mutational dynamics parallel long-term genome evolution in endosymbionts

   https://doi.org/10.1093/molbev/msag131  

   Kwak, Younghwan; Bennett, Gordon M (July 2026, Molecular Biology and Evolution)

   Agashe, Deepa (Ed.)

   Abstract Obligate endosymbionts of insects undergo extreme genome evolution, marked by accelerated molecular evolution, severe base-pair compositional bias, and massive gene loss. However, the microevolutionary processes driving these patterns remain poorly understood, as they occur at intrahost population scales that are rarely captured. To address this gap, we measured intrahost genetic diversity of two endosymbionts, Karelsulcia and Nasuia, from the aster leafhopper, Macrosteles quadrilineatus (Hemiptera: Cicadellidae). Contrary to the theoretical expectation of strict clonality, we found that both endosymbionts harbor measurable intrahost genetic variation, with lineage-specific mutational dynamics that parallel long-term evolutionary trends. Karelsulcia showed sparse intrahost variation dominated by repeat-associated indels, while Nasuia exhibited more abundant single-nucleotide mutations that appear to shape genome-wide A + T bias. Mitochondrial heteroplasmy did not covary with endosymbiont nucleotide diversity, indicating that these patterns are not driven by host-level dynamics. Notably, recurrent nonsynonymous variants in Nasuia affect essential genes for amino acid biosynthesis and translation. The intrahost mutational patterns we observed in endosymbionts are consistent with long-term sequence changes between our population founder genome and contemporary endosymbiont populations after ∼11 years of maintenance. Taken together, our results demonstrate how distinct mutational processes operating at the intrahost population scale drive macroevolutionary patterns in endosymbiont genomes. Moreover, our study establishes that laboratory endosymbiont systems provide a powerful framework for dissecting and understanding these fundamental evolutionary processes. 

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