Copy-number variants at genomic loci evolve at a high rate, are linked to many different diseases, and play a role in adaptive evolution in humans and other organisms. Here, we show that stickleback fish from freshwater environments have rapidly and repeatedly evolved an expanded number of copies of a gene family involved in muscle development, myosin heavy chain 3 cluster C (MYH3C), compared with marine populations. Differences in copy number between marine and freshwater fish are maintained even in the presence of gene flow, suggesting that MYH3C changes represent adaptive divergence between ecotypes. Copy-number expansion occurs by tandem duplication of MYH3C coding and regulatory regions on the stickleback sex chromosome. We identify a muscle regulatory enhancer within the expanded MYH3C region and show that elevated copy number is associated with developmental and tissue-specific increases in corresponding mRNA expression levels in skeletal muscle. Common MYH3C clusters include 3-, 4-, 5-, and 6-copy variants that likely evolved through a combination of microhomology-mediated break repair and non-allelic homologous recombination. Our results provide a new example of copy-number changes in a wild species and identify copy-number variations (CNVs) as potential “hotspots” of repeated adaptive evolution.

Understanding the response of phenotypically plastic traits to novel environments is critical to predicting evolutionary dynamics. "Simpson-Baldwin dynamics" refer to the expected evolutionary response of a plastic trait to a constant novel environment: an initial increase in plasticity followed by a long-term decline. While theoretically well-supported, demonstrating Simpson-Baldwin dynamics has proven elusive in natural populations-and the mechanisms underlying a predicted long-term loss of plasticity remain obscure, with no clear evidence of a universal cost to plasticity. By lab-rearing diverse, wild-caught threespine stickleback (Gasterosteus aculeatus) populations under a range of salinities and recording the plasticity of morphological, physiological, and fitness-related traits, we provide evidence of Simpson-Baldwin dynamics and insights into the underlying mechanisms. Following freshwater colonization, populations showed a short-term increase or maintenance of salinity tolerance breadth-while a subsequent loss of salinity tolerance occurred in most, but not all, populations over the longer term. Despite variability amongst physiological and morphological responses to increased salinity across populations, we find that resolution of generalist-specialist trade-offs may drive plasticity loss: less-plastic populations grew faster in freshwater. Our findings establish that Simpson-Baldwin dynamics can apply to plastic traits in natural populations, although the underlying mechanisms may be variable, even within species.

This study used a cluster randomized controlled trial to investigate the effectiveness of two approaches to increasing middle school students' science learning when using an inquiry-based science curriculum. Eighty-nine schools, with 253 teachers and 20,591 students, were randomly assigned into one of three conditions: (a) a treatment condition in which the textbook curriculum was modified based on three principles of cognitive science coupled with teacher professional development (PD), (b) a second treatment condition in which teachers received PD designed to improve their knowledge of the science content, and (c) a business-as-usual control group. The cognitive science treatment had a small but statistically significant positive effect on content learning, with a stronger impact on students of higher prior achievement. Compared to business-as-usual, the intervention to increase teacher content knowledge had no impact.

Freshwater populations of threespine stickleback (Gasterosteus aculeatus) have diverged from their marine ancestor and show extensive variation among populations throughout most of their range. However, phenotypes of freshwater populations from the east coast of North America do not appear to demonstrate extensive variability observed throughout the rest of the species' range. The relatively young age of east coast North American populations may explain the apparent lack of freshwater variability. On the other hand, populations in this part of the species' range have not received the same level of attention as populations throughout the rest of its range have, and the low level of variability observed may simply reflect a lack of data for this region. We examined morphological traits (including linear measurements of body armor and body shape) of stickleback from 52 locations representing marine and freshwater populations along the west coast of Newfoundland, Canada. We found that variability in morphological traits among freshwater populations was much more extensive than previously assumed and similar to patterns observed for populations in mainland British Columbia, Alaska, and western Europe.

Human-mediated environmental challenges are severely impacting local and global levels of biodiversity. Phenotypic plasticity can play a major role in aiding population persistence because plasticity can buffer populations from novel challenges. Given this role, it is possible for the degree of plasticity to evolve rapidly as it may place phenotypes near, but not exactly on, an adaptive peak for the new environment. Here, we asked if behavioral plasticity evolves following the introduction of a novel predator. We measured behavioral responses of threespine stickleback (Gasterosteus aculeatus) populations, with and without the introduced apex predator northern pike (Esox lucius), to three different chemical odors, (1) conspecific alarm odors, (2) digested odors, and (3) pheromones of northern pike (pike odor). We demonstrated that stickleback from pike-invaded populations do not respond to any of the treatments in our study, while stickleback from pike-free populations behaviorally respond to the presence of alarm odors but not digested or pike-specific pheromones. Our results provide evidence that the behavioral plasticity of stickleback populations can rapidly evolve to the presence of northern pike, and more importantly, that this response is consistent across multiple populations.

The cestode Schistocephalus solidus is a common parasite in freshwater threespine stickleback populations, imposing strong fitness costs on their hosts. Given this, it is surprising how little is known about the timing and development of infections in natural stickleback populations. Previous work showed that young-of-year stickleback can get infected shortly after hatching. We extended this observation by comparing infection prevalence of young-of-year stickleback from 3 Alaskan populations (Walby, Cornelius and Wolf lakes) over 2 successive cohorts (2018/19 and 2019/20). We observed strong variation between sampling years (2018 vs 2019 vs 2020), stickleback age groups (young-of-year vs 1-year-old) and sampling populations.

Invasive predators often impose devastating selection pressures on native prey species. However, their effects can be regionally dependent and influenced by the local ecological conditions of their invaded habitats. Evolved behavioral phenotypes are important mechanisms by which prey adapt to the presence of novel predators. Here, we asked how behavior and behavioral plasticity of threespine stickleback (Gasterosteus aculeatus) populations have evolved following the introduction of the invasive predator, northern pike (Esox lucius). We examined the behavior of F1 offspring generated from three pike-free and three pike-invaded populations and measured how stickleback activity and plant use behaviors, and their plasticity, have evolved following pike introduction. To evaluate plasticity, we exposed juvenile stickleback to predator cues during their first year of development and then evaluated how this repeated exposure influenced behavioral responses to an artificial predation event. We found no overarching effect of pike in either evolved behaviors or behavioral plasticity, and no evidence for the presence of developmental plasticity. Furthermore, we found that depending on the phenotype, pike-invaded stickleback populations have either more or less among-population variation than pike-free populations. Our results suggest that evolution in response to invasive predators may be hidden by local adaptation when enough populations are studied.

Predation often has consistent effects on prey behavior and morphology, but whether the physiological mechanisms underlying these effects show similarly consistent patterns across different populations remains an open question. In vertebrates, predation risk activates the hypothalamic-pituitary-adrenal (HPA) axis, and there is growing evidence that activation of the maternal HPA axis can have intergenerational consequences via, for example, maternally-derived steroids in eggs. Here, we investigated how predation risk affects a suite of maternally-derived steroids in threespine stickleback eggs across nine Alaskan lakes that vary in whether predatory trout are absent, native, or have been stocked within the last 25 years. Using liquid chromatography coupled with mass spectroscopy (LC-MS/MS), we detected 20 steroids within unfertilized eggs. Factor analysis suggests that steroids covary within and across steroid classes (i.e. glucocorticoids, progestogens, sex steroids), emphasizing the modularity and interconnectedness of the endocrine response. Surprisingly, egg steroid profiles were not significantly associated with predator regime, although they were more variable when predators were absent compared to when predators were present, with either native or stocked trout. Despite being the most abundant steroid, cortisol was not consistently associated with predation regime. Thus, while predators can affect steroids in adults, including mothers, the link between maternal stress and embryonic development is more complex than a simple one-to-one relationship between the population-level predation risk experienced by mothers and the steroids mothers transfer to their eggs.

The freshwater pearl musselMargaritifera margaritiferais an ecologically important globally endangered species, yet little is known about its biodiversity and population genetics in North American populations. This study focused onM. margaritiferafrom six locations within two stream systems in central and eastern Massachusetts, USA, to understand the historical impact of damming and habitat fragmentation on local population structure and genetic diversity. Approximately 300 informative single nucleotide polymorphisms were generated from 59 individuals across six geographical locations, using the restriction-site-associated DNA sequencing approach. Genotypes were also gleaned from publicly available RNA sequencing data of 23 FrenchM. margaritiferasamples. Predictions of population structure using discriminant analysis of principal components and fastStructure both indicate the existence of a single genetic population in central and eastern Massachusetts, and a clear separation from French individuals. This observation is further supported by frequency-based calculations of population genetics, which indicate near-complete admixture and a high number of migrants between Massachusetts sites. These results suggest that the effects of habitat fragmentation from damming in the past century have not radiated through the Massachusetts population ofM. margaritifera, potentially because of the long lifespan and generation time of the species. However, the Massachusetts population has similar levels of genetic diversity to the endangered populations in Europe, and these factors should be considered by conservationists when creating conservation and management plans. The findings of this study suggest that populations in North America are reasonably healthy and diverse and that, as one population, the study area represents one conservation unit. This indicates that dam removal, habitat restoration, and captive breeding programmes may be viable options to increase the small population numbers and retain a substantial portion of genetic diversity without sacrificing local adaptation.

Behavioural phenotypes are notable for their plasticity, in that individual behaviour patterns can be expressed ephemerally in response to an appropriate stimulus and then disappear. In the absence of an appropriate stimulus, behavioural phenotypes can remain unexpressed over many generations, yet the capacity to perform the behaviour can be retained. Here we discuss potential evolutionary influences of unexpressed behavioural phenotypes using two examples, one from the post-glacial adaptive radiation of the threespine stickleback fish, Gasterosteus aculeatus, and one from a far more ancient radiation, that of Pheidole ants. These radiations demonstrate that unexpressed phenotypes can persist for thousands or more generations in a condition that permits re-expression when an appropriate stimulus appears in the environment. We describe possible explanations for persistence of unexpressed phenotypes and demonstrate that in the absence of an appropriate phylogeny, re-expressed traits could be interpreted as true novelties in the group. We then discuss the way in which loss of expression can lead to parallelism in adaptive radiations — a mechanism that is rarely considered. We conclude by examining the way in which reappearance of unexpressed ancestral traits has the potential to facilitate population persistence if ancestral environments reappear and discuss the reasons that the evolutionary implications of behavioural plasticity are not better studied.

Include 9 exemplar video tapes of the display types by ecotype