Biology subjects

Iverson, E. N. K.

Publications and source records attributed to Iverson, E. N. K..

5 recordsLinked to original sources

bioRxiv2026

Adaptation to increasing elevation via metabolic rates and mitochondrial genetics in Andean birds

Closely related species often replace each other at different elevations, a pattern that results in high biodiversity in mountains. Species range limits are set by physiological adaptation to gradients in temperature and oxygen availability as well as by competition with species above and below them on the gradient. As mountains warm due to climate change and many species move upward in elevation it is increasingly important to understand the mechanisms of climatic adaptation and competition that govern where and when species will persist, migrate, or go extinct. We investigated the genetics and physiology of tapaculos (Passeriformes: Rhinocryptidae), a group of small, weakly flying birds known for their narrow elevation ranges across the Andes, where as many as 6 species may replace each other on one transect. We first looked for selection on the mitochondrial genome, which is thought to be important in adaptation to elevation, in 85 members of this clade across South America. We found a signature of intensified positive selection on mitochondrial genes among species with mid-point elevations above 3000 m, suggesting metabolic adaptation to the harsh, high-altitude environment. We then measured metabolic rates and mitochondrial function in the field in four species of the genus Scytalopus that replace each other in a series along the Manu elevation gradient in Southeastern Peru. We found that higher elevation species have higher Basal Metabolic Rates (BMR) and more extreme Lower Critical Temperatures (LCTs), suggesting adaptation to the cold. However, these differences do not have a clear basis in mitochondrial function, which was broadly similar across species. Rather, higher-elevation species are smaller overall but have larger relative organ masses and maintain lower body temperatures, which may be adaptations to meeting the high thermogenic demands of their environment. These four species conform well to expectations of the Competitive-Exclusion Tolerance Rule (Martin & Ghalambor, 2023), with each species up the gradient being a poorer competitor (smaller body size) but having adapted to a harsher part of the gradient that more competitive species cannot occupy. We discuss these results in the context of theories about the importance of climate, competition, and ecotones in the distribution of Andean organisms, predicting that species living in the highest-elevation forests just below the treeline may be at the greatest risk of extinction due to climate change.

evolutionary biology↗

bioRxiv2026

Opposing behavioral roles for a single gene in a species with a supergene polymorphism

Behavioral trade-offs, such as between territorial aggression and parental care, require coordination among neural and genetic mechanisms that influence opposing phenotypes. In some systems, including the white-throated sparrow, this coordination has been proposed to be mediated in part by chromosomal inversions that bind together multiple genes associated with alternative strategies. Here, we demonstrate that opposing components of a life-history trade-off can be linked, via alternative alleles inside a chromosomal rearrangement, to a single pleiotropic gene. In white-throated sparrows, a species with polymorphic aggressive and parental behavior, the gene encoding the neuromodulator vasoactive intestinal peptide (VIP) lies within an inversion-based supergene. Working with a free-living population, we found that VIP expression in a pro-aggression cell population in the anterior hypothalamus positively predicted territorial singing, whereas expression in a pro-parental cell population in the infundibular nucleus positively predicted parental provisioning. Expression in these two regions was negatively correlated, consistent with opposing behavioral roles. Allele-specific analyses revealed stronger expression bias toward the supergene-associated allele in the anterior hypothalamus than in the infundibular nucleus. These findings suggest that chromosomal inversions can support opposing behavioral phenotypes not only by linking multiple genes, but also by driving regulatory divergence of individual genes that organize behavioral trade-offs.

neuroscience↗

bioRxiv2025

Admixture mapping reveals evidence for multiple mitonuclear incompatibilities in swordtail fish hybrids

How barriers to gene flow arise between closely related species is one of the oldest questions in evolutionary biology. Classic models in evolutionary biology predict that negative epistatic interactions between variants in the genomes of diverged lineages, known as hybrid incompatibilities, will reduce viability or fertility in hybrids. The genetic architecture of these interactions and the evolutionary paths through which they arise have profound implications for the efficacy of hybrid incompatibilities as barriers to gene flow between species. While these questions have been studied using theoretical approaches for several decades, only recently has it become possible to genetically map larger numbers of hybrid incompatibilities. Here, we use admixture mapping in natural hybrid populations of swordtail fish (Xiphophorus) to identify hybrid incompatibilities involving genetic interactions between the mitochondrial and nuclear genomes. We find that at least nine regions of the genome are involved in mitonuclear incompatibilities. These incompatibilities involve interactions between the nuclear genome and the X. malinche mitochondria, the X. birchmanni mitochondria, or both. Moreover, they vary in the strength of selection they experience, and the degree to which they limit gene flow in natural hybrid populations. Our results build a deeper understanding of the complex architecture of selection against incompatibilities in naturally hybridizing species and highlight an important role of mitonuclear interactions in the evolution of reproductive barriers between closely related species.

evolutionary biology↗

bioRxiv2024

Stronger evidence for relaxed selection than adaptive evolution in high-elevation animal mtDNA

Mitochondrial (mt) genes are the subject of many adaptive hypotheses due to the key role of mitochondria in energy production and metabolism. One widespread adaptive hypothesis is that selection imposed by life at high elevation leads to the rapid fixation of beneficial alleles in mtDNA, reflected in the increased rates of mtDNA evolution documented in many high-elevation species. However, the assumption that fast mtDNA evolution is caused by positive, rather than relaxed purifying selection has rarely been tested. Here, we calculated the dN/dS ratio, a metric of nonsynonymous substitution bias, and explicitly tested for relaxed selection in the mtDNA of over 700 species of terrestrial vertebrates, freshwater fishes, and arthropods, with information on elevation and latitudinal range limits, range sizes, and body sizes. We confirmed that mitochondrial genomes of high-elevation taxa have slightly higher dN/dS ratios compared to low-elevation relatives. High-elevation species tend to have smaller ranges, which predict higher dN/dS ratios and more relaxed selection across species and clades, while absolute elevation and latitude do not predict higher dN/dS. We also find a positive relationship between body mass and dN/dS, supporting a role for small effective population size leading to relaxed selection. We conclude that higher mt dN/dS among high-elevation species is more likely to reflect relaxed selection due to smaller ranges and reduced effective population size than adaptation to the environment. Our results highlight the importance of rigorously testing adaptive stories against non-adaptive alternative hypotheses, especially in mt genomes.

evolutionary biology↗

bioRxiv2022

Disentangling positive vs. relaxed selection in animal mitochondrial genomes

Disentangling different types of selection is a common goal in molecular evolution. Elevated dN/dS ratios (the ratio of nonsynonymous to synonymous substitution rates) in focal lineages are often interpreted as signs of positive selection. Paradoxically, relaxed purifying selection can also result in elevated dN/dS ratios, but tests to distinguish these two causes are seldomly implemented. Here, we reevaluated seven case studies describing elevated dN/dS ratios in animal mtDNA and their accompanying hypotheses regarding selection. They included flightless vs. flighted lineages in birds, bats, and insects, and physiological adaptations in snakes, two groups of electric fishes, and primates. We found that elevated dN/dS ratios were often not caused by the predicted mechanism, and we sometimes found strong support for the opposite mechanism. We discuss reasons why energetic hypotheses may be confounded by other selective forces acting on mtDNA and caution against overinterpreting molecular "spandrels", including elevated dN/dS ratios.

evolutionary biology↗