Browsing by Author "Hufbauer, Ruth A., advisor"

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Open Access
Adaptation to new and changing environments: from experiments to field studies
(Colorado State University. Libraries, 2024) Durkee, Lily F., author; Hufbauer, Ruth A., advisor; Ruegg, Kristen, committee member; Angeloni, Lisa, committee member; Melbourne, Brett A., committee member
In today's rapidly changing world, organisms are often exposed to new or challenging environments. Following environmental change, populations may experience one or more of the following outcomes: they may disperse to a new habitat, persist through phenotypic plasticity or adaptive evolution, or go extinct. Here, I focus on adaptive evolution as a mechanism for enabling populations to respond to different types of environmental change. Adaptation may be necessary in less mobile species, or in fragmented landscapes where dispersal corridors are not available. Insects can serve as excellent models for studying the evolutionary dynamics of populations, as their life cycles are relatively short and multiple generations can occur within a few years, enabling rapid adaptation. In this dissertation, I explore the impacts of a change in phenology (Chapter 1), a challenging environment (Chapter 2), and the presence of a mountain range (Chapter 3) on the adaptation of insect populations in both the laboratory (Chapters 1-2) and in the field (Chapter 3). To begin, I studied the effects of a phenology shift using experimental evolution. Climate change can affect the length and timing of seasons, which in turn can alter the time available for insects to complete their life cycles and successfully reproduce. Intraspecific hybridization between individuals from genetically distinct populations, or admixture, can boost fitness in populations experiencing environmental challenges. Admixture can particularly benefit small and isolated populations that may have high genetic load by masking deleterious alleles, thereby immediately increasing fitness, and by increasing the genetic variation available for adaptive evolution. To evaluate the effects of admixture on populations exposed to a novel life cycle constraint, I used the red flour beetle (Tribolium castaneum) as a model system. Distinct laboratory lineages were kept isolated or mixed together to create populations containing 1 to 4 lineages. I then compared the fitness of admixed populations to 1-lineage populations while subjecting them to a shortened generation time. After an initial decline in fitness in the new environment, the admixed populations demonstrated significantly greater fitness compared to the 1-lineage populations after three generations. The timing of the increase in fitness suggests that adaptation to the novel environmental constraint occurred, as opposed to the masking of deleterious alleles. In Chapter 2, I examined the effects of consistent immigration on the probability and timing of adaptation. Theory predicts that immigration can delay extinction and provide novel genetic material that can prevent inbreeding depression and facilitate adaptation. However, when potential source populations have not experienced the new environment before (i.e., are naive), immigration can counteract selection and constrain adaptation. This study evaluated the effects of immigration of naive individuals on adaptation in experimental populations of red flour beetles. Small populations were exposed to a challenging environment, and three immigration rates (0, 1, or 5 migrants per generation) were implemented with migrants from a benign environment. Following an initial decline in population size across all treatments, populations receiving no immigration gained a higher growth rate one generation earlier than those with immigration, illustrating the constraining effects of immigration on adaptation. After seven generations, a reciprocal transplant experiment found evidence for adaptation regardless of immigration rate. Thus, while the immigration of naive individuals briefly delayed adaptation, it did not increase extinction risk or prevent adaptation following environmental change. For Chapter 3, I shifted my focus from laboratory populations to natural populations. In nature, elevation gradients provide a way to study populations across a range of environmental conditions within relatively small spatial scales. Adaptation to the local habitat might occur in response to the unique selection pressures present at different elevations, resulting in distinct populations adapted to different environments. However, if there is high habitat connectivity, gene flow can slow or prevent adaptation while also maintaining genetic variation and large population sizes. In this chapter, I used genomics to investigate the interplay between selection and gene flow in butterfly populations collected from high (>2,500m) and low (<2,000m) elevations, both east and west of the Rocky Mountains. My study focused on the Rocky Mountain subspecies of the clouded sulfur butterfly (Colias philodice eriphyle Edwards). Weak, but statistically significant, patterns of population differentiation were apparent between butterflies collected east and west of the Rockies, and eastern butterflies harbored greater genetic diversity compared to those from the west. Additionally, FST values close to zero suggest gene flow was high among the collection sites. I used a redundancy analysis to show that the observed east-west differentiation may be largely driven by greater precipitation east of the Rockies, and I identified over 16,000 putatively adaptive loci and 3,000 candidate genes associated with the environmental variables that may underly adaptive traits. In summary, my dissertation research highlights that adaptation can occur with genetic mixing or immigration in less than 10 generations in the lab (Chapters 1-2) and that adaptation to different environments can be identified among well-connected populations in the field (Chapter 3). It is well documented that gene flow can help maintain genetic variation, particularly among fragmented populations. My results emphasize that gene flow does not impede natural selection (Chapters 2-3), and that mixing between distinct populations can even promote adaptation (Chapter 1). My findings, therefore, support the use of methods such as translocation and wildlife corridors to facilitate gene flow for sexually reproducing conservation targets.
Open Access
Ecology and plant defense of two invasive plants, Hyoscyamus niger and Verbascum thapsus
(Colorado State University. Libraries, 2016) Fettig, Christa E., author; Hufbauer, Ruth A., advisor; McKay, John K., committee member; Norton, Andrew P., committee member; Savidge, Julie A., committee member
Understanding the factors that drive non-native plant populations to succeed in a new range and the ecological and biological differences that set introduced populations apart from their native counterparts can provide insight into ecological and evolutionary processes, as well as information crucial to management. In this dissertation, I present research on two different plant species that have been introduced to North America, both of which can now be found across the United States and throughout Canada. Chapters 1 and 2 focus on Hyoscyamus niger (black henbane, Solanaceae), a poisonous and state-listed noxious weed. In chapter one I experimentally evaluate whether introduced populations in the western United States are annual or biennial. Both of these life cycles are found in the native range, and have a clear genetic basis. I experimentally manipulated vernalization (a cold treatment for 19 weeks), and find that plants in the introduced range are biennial. Vernalization is critical for bolting and flowering to occur within a growing season. Interestingly, given enough time in a greenhouse setting, 26 percent of plants that were not vernalized were able to flower. This is unlikely to happen in nature, however, as warmer regions without a cold period to naturally vernalize plants are typically lacking sufficient resources (e.g. adequate water or space) for this species. Chapter two aims to understand basic biological and ecological characteristics of black henbane in the introduced range, which lays the groundwork for additional ecological and evolutionary research on this species and will also help direct appropriate management practices. In a greenhouse experiment, I test the effects of selfing and outcrossing. In field populations, I measure reproductive output, the size of seed banks of introduced populations, the viability of seed collected over four years, patterns of mortality, and fluctuation in the size of 15 populations. Black henbane is self-compatible, and capable of producing copious seed, and generating large seed banks in naturalized populations. Seeds remain viable for multiple years which may contribute to the dynamic fluctuations of field population sizes that were observed over four years. Populations are generally ephemeral, with high mortality at the rosette stage. Chapter 3 is focused on resistance and tolerance to herbivory, and how they might vary between ranges as well as within individual plants as predicted by optimal defense theory. Optimal defense predicts that defenses are allocated to different tissues based on their value to the plant. I use Verbascum thapsus (common mullein, Scrophulariaceae) to evaluate resistance to both a specialist and a generalist herbivore among plants from the native and introduced range and among leaves of different ages. I also measure tolerance to defoliation by simulating three levels of herbivory and evaluating the regrowth of above and below ground biomass. Both native and introduced mullein plants are highly defended against specialist and generalist herbivores, with high levels of both resistance and tolerance. In accordance with optimal defense theory, young leaves are more highly defended than older leaves.
Open Access
Evolutionary and chemical ecology of Verbascum thapsus reveal potential mechanisms of invasion
(Colorado State University. Libraries, 2011) Alba, Christina, author; Hufbauer, Ruth A., advisor; Detling, James K., committee member; Bowers, M. Deane, committee member; Knapp, Alan K., committee member
Biological invasions, which occur when introduced species achieve pest status due to dramatic increases in performance, cause substantial environmental and economic damage. Invasion dynamics are extremely complex, varying in space and time, and as a function of the associations that form between introduced species and the biota present in the communities they invade. For plants, herbivores play a central role in shaping the outcome of introduction events. In particular, when plants are introduced to novel ranges, they often leave behind coevolved specialist herbivores (typically insects) that act to suppress populations in the native range. This can lead to increases in plant performance, for example when introduced plants evolving in communities devoid of enemies reallocate resources from defenses to growth and reproduction. Because of the important biological associations that exist between plants and insect herbivores, as well as the dramatic shifts in these associations that characterize biological invasions, this research places a particular emphasis on the evolutionary and chemical ecology of plant-insect interactions. More broadly, this research quantifies several aspects of the invasion dynamics of the introduced weed Verbascum Thapsus L. (Scrophulariaceae, common mullein). I first present data from a biogeographic comparison in which a survey of more than 50 native (European) and introduced (United States) mullein populations confirms a marked increase in population- and plant-level performance in the introduced range. I also document several ecological differences between ranges, including shifts in the abundance, identity, and degree of damage caused by insect herbivores, as well as differences in the abundance and identity of plant competitors and precipitation availability. A greenhouse experiment revealed that the increased performance observed in the field is maintained when native and introduced plants are grown from seed in a common environment; thus, a component of the performance phenotype is genetically based, or evolved. However, this increase in performance is not associated with an evolved decrease in defense investment as predicted by the evolution of increased competitive ability (EICA) hypothesis. Indeed, despite significant population-level variation in several defenses (trichomes, leaf toughness and iridoid glycosides), there is no evidence for the evolution of range-level differences in defense investment. I further explored how mullein's investment in chemical defense varies in natural populations and in relationship to damage by chewing herbivores. Based on this exploration, I developed new predictions for how changes to defense allocation may result in increased performance. Natural mullein populations exposed to ambient levels of herbivory in the introduced range exhibit significant population- and plant-level variation in iridoid glycosides. In particular, young (highly valuable) leaves are more than 6 better defended than old leaves, and likely because of this incur minimal damage from generalist herbivores. The limited ability of generalists to feed on mullein's well-defended young leaves results in negligible losses of high-quality tissue, suggesting a mechanism for mullein's increased performance in North America. Indeed, the within-plant distribution of iridoid glycosides significantly differs between native and introduced plants exposed to the different insect communities present in each range. Importantly, introduced mullein invests significantly more in the chemical defense of valuable young leaves than does native mullein, which leads to a dramatic reduction in the attack of young leaves in the introduced range relative to the native range. This optimization of within-plant investment in defense reflects the fact that introduced mullein has been released from the evolutionary dilemma posed by simultaneous attack by specialist and generalist herbivores (with specialists often being attracted to the same chemicals used to deter generalists from feeding, resulting in stabilizing selection on defense levels). In summary, this research provides evidence for a dramatic increase in the performance of introduced common mullein that is associated with several ecological differences between ranges as well as potentially adaptive shifts in mullein's chemical defense investment under natural conditions.
Open Access
Frameworks for testing mechanisms of invasion and plant defense
(Colorado State University. Libraries, 2018) Endriss, Stacy B., author; Hufbauer, Ruth A., advisor; Norton, Andrew P., advisor; Bowers, M. Deane, committee member; Ghalambor, Cameron K., committee member
To view the abstract, please see the full text of the document.
Open Access
The roles of phenotypic plasticity and adaptation in morphology and performance of an invasive species in a novel habitat
(Colorado State University. Libraries, 2020) Jardeleza, Marcel Kate Guarin, author; Hufbauer, Ruth A., advisor; Pearse, Ian S., committee member; Pejchar, Liba, committee member; Ghalambor, Cameron K., committee member
Invasive species spread and thrive across widely variable habitats. Their success in novel environments may be influenced by phenotypic plasticity, which occurs when a genotype can produce multiple phenotypes in response to different environments, or local adaptation, the production of traits that are advantageous under the local environmental conditions regardless of their effects in other habitats. One indication of these non-mutually exclusive processes comes in the form of geographic or elevational clines in phenotypes and genotypes. Drosophilla suzukii is an outstanding example of an invasive species that has established across many diverse environments and exhibits an elevational cline in wing size. In my thesis, with collaborators Jonathan Koch, Ian Pearse, Cameron Ghalambor, and Ruth Hufbauer, I evaluated the degree to which plasticity and genetic differentiation determine differences in wing sizes, and whether plasticity appears to be adaptive or not. I first characterized an elevational cline in wing size in D. suzukii on Hawaii and also evaluated its relative abundance by elevation. I then conducted a reciprocal temperature experiment to understand the mechanisms driving the cline. We found that wing size increased with elevation and that D. suzukii was significantly more abundant in higher elevation sites compared to lower elevation sites. Temperature may be the key driver of wing size variation, with wing size increasing as temperature decreased along the elevational gradient. In the reciprocal temperature experiment, I found that temperature had a strong effect on development time and cooler temperatures took longer to emerge compared to warmer temperatures. The reciprocal temperature experiment further revealed strong phenotypic plasticity. When flies from high and low elevation were reared at a cool temperature comparable to that found at high elevation, they produced larger wings. When reared at a warm temperature comparable to that found at low elevation, they produced smaller wings, which is the same pattern of variation observed in field populations. Additionally, I found significant differences in the number of flies that emerged from the two experimental temperatures. Flies from low and high elevation sites produced similar numbers of offspring at the cool temperature, while high elevation flies produced significantly more offspring at the warm temperature compared to the low elevation flies, despite that temperature being their home temperature. My study revealed strong plasticity in wing size, but no indication of local adaptation. If the wing phenotypes observed in high and low elevation populations in the field represent fit phenotypes, then this plasticity is adaptive. The flies may be exhibiting an "all-purpose genotype" where a fit phenotype is produced across the environmental conditions and there is no selection for adaptation to occur. As evidence continues to mount in support of the highly plastic responses of D. suzukii to temperature, particularly with respect to wing size, and the possible adaptiveness of this response, future studies need to make the direct connection between wing plasticity and adaptation. How an invasive organism responds to different environments determines the extent of its novel range and the places that it will impact. Hawaiian populations of D. suzukii exhibit substantial phenotypic variation in wing size, development time, and offspring production with some genetic component to that plasticity.