Mountain Scholar :: Browsing by Author "Funk, W. Chris, advisor"

Browsing by Author "Funk, W. Chris, advisor"

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Open Access
Diversity, distributions, and evolution of Rocky Mountain and Andean stream insects
(Colorado State University. Libraries, 2017) Gill, Brian Andrew, author; Funk, W. Chris, advisor; Kondratieff, Boris C., advisor; Poff, N. LeRoy, committee member; Clements, Will H., committee member
Concordant with latitudinal increases in seasonality, the "climate variability hypothesis" (CVH) posits that the breadth of species' thermal tolerances should increase with latitude. Across elevations, the "mountain passes are higher in the tropics hypothesis" (MPHT) postulates that the narrow thermal tolerances of tropical species should limit their dispersal across elevations more than the broad thermal tolerances of temperate species. We consequently expect tropical species to have more limited elevational ranges and higher rates of population isolation, divergence, and speciation than temperate species, which could lead to higher tropical than temperate species richness. Moreover, many tropical species might be cryptic, as they have diverged primarily in physiological and dispersal traits, rather than traits with distinct morphological phenotypes. In this dissertation, I investigate how the CVH might provide a mechanistic explanation for global trends in species richness, cryptic diversity, and elevational distributions. In chapter one, I recapitulate the CVH and MPHT hypothesis and summarize related key literature. In chapter two, I characterize the diversity and distributions of stream insects in the Colorado Rocky Mountains. In chapter three, I compare the species richness and elevational ranges of species from the Colorado Rocky Mountains and the Andes of Ecuador. Lastly in chapter four, I integrate data from physiological, landscape genetic, and biogeographic investigations to evaluate the support for the CVH as a key mechanism determining global trends in species diversity, distributions, and vulnerability to climate change.
Open Access
Effects of hydroperiods and predator communities on Pseudacris maculata: a model species for climate change impacts on amphibians
(Colorado State University. Libraries, 2013) Amburgey, Staci Marie, author; Funk, W. Chris, advisor; Murphy, Melanie, committee member; Muths, Erin, committee member; Bailey, Larissa, committee member; Poff, LeRoy, committee member
To view the abstract, please see the full text of the document.
Open Access
Evolutionary underpinnings of microgeographic adaptation in song sparrows distributed along a steep climate gradient
(Colorado State University. Libraries, 2021) Gamboa, Maybellene Pascual, author; Ghalambor, Cameron K., advisor; Funk, W. Chris, advisor; Sillett, T. Scott, committee member; Wolf, Blair O., committee member; Hufbauer, Ruth A., committee member; Morrison, Scott A., committee member
Understanding how evolutionary processes interact to maintain adaptive variation in natural populations has been a fundamental goal of evolutionary biology. Yet, despite adaptation remaining at the forefront of evolutionary theory and empirical studies, there remains a lack of consensus about the evolutionary conditions that enable adaptation to persist in natural populations, especially when considering complex phenotypes in response to multivariate selection regimes. In my dissertation, I disentangle the evolutionary mechanisms that shape adaptive divergence in song sparrows (Melospiza melodia) distributed along a climate gradient on the California Channel Islands and nearby coastal California. First, I found evidence that climate, and neither vegetation nor selection for increased foraging efficiency, likely drive adaptive divergence in bill morphology among insular populations. Second, I used an integrated population and landscape genomics approach to infer that bill variation is indicative of microgeographic local adaptation to temperature. Lastly, I tested whether the distinct climate gradient facilitates adaptative divergence in other thermoregulatory traits and found evidence to support environmental temperatures result in fixed population differences in many complementary phenotypes, including plumage color, feather microstructure, and thermal physiology. Collectively, these results find support for microgeographic climate adaptation in a suite of complex phenotypes and demonstrate the utility of integrative approaches to infer local adaptation in natural populations. Finally, by developing a more holistic understanding of climate adaptation in natural populations, my results inform conservation management of this species of special concern.
Open Access
Linking organismal physiology and the landscape to predict vulnerability to climate change
(Colorado State University. Libraries, 2023) Cicchino, Amanda Stephanie, author; Funk, W. Chris, advisor; Ghalambor, Cameron, committee member; Kanno, Yoichiro, committee member; Hoke, Kim, committee member; Landguth, Erin, committee member
Global temperatures continue to increase at unprecedented rates, both in mean and in variance. Thus, a major challenge for scientists of the 21st century is to predict whether species will persist through these changes. One way to partly assess vulnerability to climate change is to investigate the relationships between the environment and traits that are either particularly sensitive to temperature or may confer resilience against thermal changes. In ectotherms, external temperatures dictate their physiology, thus thermal physiological traits may be key to understanding ectothermic persistence. Although population variation is integral to the evolvability of thermal physiological traits, most studies using these traits to infer vulnerability extrapolate data from one or few populations to represent the species. Furthermore, many studies also use coarse metrics of environmental temperatures which may not fully capture the variation experienced by the organism. Here, using a cold-water frog system, I demonstrate the relationships between thermal physiological traits and local environmental temperatures among populations. In my first chapter, I provide a brief overview of ectothermic physiology, environmental thermal landscapes, and the ecology of the two species of tailed frogs that I investigated. In my second chapter, I show that populations of tailed frogs vary in their critical thermal limit (CTmax) plasticity, which impacts species-level assessments of vulnerability. I also demonstrate the methodological impacts of ignoring acute responses to temperature when estimating plasticity in this trait. For my third chapter, I demonstrate relationships between CTmax and local thermal environments, including temporal and spatial variability in temperature, among populations of tailed frogs. These results show that tailed frogs have limited opportunity for behavioural avoidance of warm temperatures, and that populations of one tailed frog species show a positive relationship between CTmax and maximum stream temperature while populations of the other species does not. In my fourth chapter, I test the critical assumption that CTmax is related to fitness, specifically mortality in ecologically relevant temperatures. My results show that populations with higher estimates of CTmax experience less mortality from thermal stress in temperatures experienced in nature, demonstrating the link between CTmax and fitness. Lastly, in my fifth chapter, I return to the plasticity in CTmax results and demonstrate the relationship between this trait and local thermal environments, showing that populations experiencing greater temperature fluctuations have greater estimates of plasticity in CTmax. Overall, these results underscore the importance of sampling widely among populations when inferring vulnerability to climate changes from physiological traits. The population variation in CTmax and its plasticity that I uncovered demonstrate the differing trends in vulnerability to climate change for the two species investigated. This work also highlights the importance of quantifying local thermalscapes and highlight how similar environments can differentially shape physiological tolerance and patterns of vulnerability among populations, in turn impacting vulnerability to future warming.
Open Access
Mechanisms of population divergence along elevational gradients in the Tropics
(Colorado State University. Libraries, 2016) Vacas, Mónica Isabel Páez, author; Funk, W. Chris, advisor; Angeloni, Lisa M., committee member; Crooks, Kevin R., committee member; Ghalambor, Cameron K., committee member; Guayasamin, Juan Manuel, committee member
Elucidating the mechanisms that give rise to population divergence and eventually initiate speciation is a key step for understanding the evolution of biodiversity. Most theories of differentiation and speciation have traditionally focused on geographically isolated populations. However, there is growing evidence that speciation can occur due to divergent selection despite initially high gene flow. My doctoral dissertation project investigates the effects of environmental heterogeneity and geography in promoting phenotypic and genetic divergence along elevation gradients in natural populations of a poison frog, Epipedobates anthonyi, across the landscape with a focus on environmental variation along elevational gradients. I studied populations distributed along a broad elevational gradient (0–1800 m above sea level) on the western slope of the Andes of southern Ecuador. First, I examined the relative roles of geographic distance and environmental gradients on genetic and phenotypic divergence. I found that populations are phenotypically divergent in size, color, and male advertisement calls, but they exhibit low genetic divergence at neutral loci. There is substantial gene flow between populations throughout the lowlands, but populations at higher elevations are relatively isolated. This is mainly due to a mountain ridge acting as a physical and possibly environmental barrier between northern and southern populations. Within elevational gradients, geographic distance corrected for topography is the main factor explaining both genetic and phenotypic divergence. However, when controlling for the effect of topographic distance, environmental conditions, such as temperature and precipitation between sites best explain observed patterns of genetic divergence, whereas environmental conditions at a given site best explains differences in phenotypic traits, presumably due to divergent selection pressures. To study the effect of temperature variation along elevational transects on adaptive divergence, I measured thermal tolerance of tadpoles across elevation. I found that populations from higher elevation had higher cold tolerance, suggesting that changes in temperature along elevation may cause divergent selection in thermal tolerance. Additionally, tadpoles from all sites have the ability to shift their thermal tolerance in response to previous exposure to different temperatures. Finally, to examine the degree of local adaptation to environmental conditions at high and low elevations, I conducted a reciprocal transplant experiment. I evaluated populations from high and low elevations from two elevational transects. Overall, I found that all populations have higher reproduction rates at low elevation. In fact, at high elevation, populations had very low reproductive rates or did not reproduce at all. However, variation in life-history traits differed between transects. Populations from one transect revealed a pattern that was consistent with the expectation under local adaptation, namely, low elevation frogs had higher reproduction than high elevation frogs at the low elevation site. In contrast, populations from the other transect had a pattern that would be expected under countergradient variation, namely higher elevation frogs had higher reproduction at the low elevation site. Intriguingly, low elevation frogs had overall higher reproduction rates than high elevation frogs, suggesting that frogs from low elevation have higher fecundity than their counterparts at high elevation. Overall, the findings of my dissertation suggest that (i) phenotypic divergence occurs in the face of gene flow, (ii) environmental variation along elevation, particularly temperature, is a force that drives population divergence, and (iii) the influences of environmental conditions on populations are variable at the intraspecific level.
Open Access
Testing the effects of gene flow on adaptation, fitness, and demography in wild populations
(Colorado State University. Libraries, 2015) Fitzpatrick, Sarah Warner, author; Funk, W. Chris, advisor; Angeloni, Lisa M., committee member; Angert, Amy L., committee member; Bailey, Larissa L., committee member; Ghalambor, Cameron K., committee member
Gene flow should reduce differences among populations, potentially limiting adaptation and population growth. But small populations stand to benefit from gene flow through genetic and demographic factors such as heterosis, added genetic variation, and the contribution of immigrants. Understanding the consequences of gene flow is a longstanding and unresolved challenge in evolutionary biology with important implications for conservation of biodiversity. My dissertation research addresses the importance of gene flow from evolutionary and conservation perspectives. In the first study of my dissertation I characterized natural patterns of gene flow and genetic diversity among remaining populations of Arkansas darters (Etheostoma cragini) in Colorado, an endemic to drying streams of the Great Plains, and a candidate for listing under the US Endangered Species Act. I found low diversity and high isolation, especially among sites with low water availability, highlighting this as a species that might eventually benefit from a well-managed manipulation of gene flow. I then turned to the Trinidadian guppy system to test the effects of gene flow using a model species for studying evolution in natural populations. My work capitalized on a series of introduction experiments that led to gene flow from an originally divergent population into native recipient populations. I was able to characterize neutral genetic variation, phenotypic variation, and population size in two native populations before the onset of gene flow. The goal of my first study using this system was to evaluate the level of gene flow and phenotypic divergence at multiple sites downstream from six introduction sites. I found that traits generally matched expectations for local adaptation despite extensive homogenization by gene flow at neutral loci, suggesting that high gene flow does not necessarily overwhelm selection. I followed up on this study by measuring many of the same traits in a common garden environment before and after gene flow to test whether gene flow caused genetically based changes in traits, and to evaluate the commonly held 'gene flow constrains divergence' hypothesis versus the 'divergence in the face of gene flow' hypothesis. I found that gene flow caused most traits to evolve, but whether those changes constrained adaptation depended on initial conditions of the recipient population. Finally, to link gene flow to changes in fitness and demography I conducted a large-scale capture-mark-recapture survey of two native populations beginning three months prior and following 26 months after upstream introductions took place. I genotyped all individuals from the first 17 months of this study to compare the relative fitness (survival and population growth rate) of native, immigrant, and hybrid guppies. In total this survey spanned 8-10 guppy generations and documented substantial increases in genetic variation and population size that could be attributed to gene flow from the introduction site. As a whole, the results from my research suggest that gene flow, even from a divergent population, can provide major demographic benefits to small populations, without necessarily diminishing locally important traits.
Open Access
The evolutionary ecology of aquatic insect range limits: a mechanistic approach using thermal tolerance
(Colorado State University. Libraries, 2018) Shah, Alisha Ajay, author; Ghalambor, Cameron K., advisor; Funk, W. Chris, advisor; Poff, N. LeRoy, committee member; Clements, William H., committee member
Understanding the effect of climate variability on species physiology and distribution is a longstanding and largely unresolved challenged in evolutionary ecology with important implications for vulnerability to climate change. My dissertation is focused on understanding the effects of temperature on physiological traits and genetic population structure of aquatic insects, to better understand the mechanisms that underlie their elevation range distributions. For my first chapter, I tested the hypothesis proposed by Dan Janzen in 1967, that temperate mountain species should have broad thermal tolerances thus allowing them to disperse easily across elevation, unhindered by the novel temperatures they encounter. On the other hand, tropical species should exhibit narrower thermal tolerances in response to the stable climate they experience. They should be physiologically challenged to disperse and be restricted to a narrow elevation range distribution. I measured critical thermal limits (CTMAX and CTMIN) and thermal breadth (difference between CTMAX and CTMIN) in several phylogenetically related temperate (Colorado) and tropical (Ecuador) aquatic insect species. I found that, as predicted, species that encounter wider stream temperature ranges, such as temperate species and high elevation tropical species, have broader thermal breadths compared to their tropical and low elevation relatives. Next, I tested how plastic the critical thermal maximum (CTMAX) response was in a subset of aquatic insects. Greater acclimation ability is thought to allow species to withstand the large temperature fluctuations associated with different seasons. Implicit in Janzen's hypothesis, is the assumption that temperate species have greater acclimation ability compared to tropical species. My experiments revealed that temperate and high elevation tropical mayfly species had greater acclimation ability compared to their relatives. However, we found no differences in acclimation capacity in stoneflies. Temperature may therefore not affect all species equally, and species acclimation ability may be a result of other factors such as body shape and evolutionary history. I then measured a third trait, metabolic rate, to investigate how it varies with temperature in temperate and tropical mayflies. Metabolic rate is arguably one of the most important traits for species because it determines the amount of energy an animal has available for its activities. I found that metabolic rates vary between temperate and tropical mayflies, and that temperatures away from a certain optimum are stressful and sometimes lethal for tropical but not temperate mayflies. Finally, I linked thermal tolerance to dispersal by correlating gene flow among populations with pairwise differences in the physiological trait CTMAX. Analyses revealed that there was lower gene flow (higher FST) among populations in Ecuador than among populations in Colorado. Within Ecuador, differences in CTMAX were highly correlated with maximum stream temperature, which was found to best explain tropical mayfly genetic structure. In Colorado, no environmental or physiological variable was found to explain population structure. Our results indicate, as Janzen predicted, that temperature can act as a significant barrier to dispersal among tropical populations but not in temperate ones. Thermal sensitivity measured as CTMAX was also correlated with FST but was not significant. As a whole, the results from my research lend support to Janzen's hypothesis and suggest that temperature plays an important role in determining range limits of aquatic insect species through its effect of thermal tolerance traits. While this research addresses long standing questions in ecology and evolution, it also has conservation implications. Most importantly, as the effects of global climate change augment, the thermally sensitive tropical species from this study system are at particular risk for extreme population declines or even extinction.
Embargo
The genomics of habitat-linked microgeographic adaptation in an island endemic bird
(Colorado State University. Libraries, 2024) Cheek, Rebecca G., author; Ghalambor, Cameron K., advisor; Funk, W. Chris, advisor; Sillett, T. Scott, committee member; Aubry, Lise M., committee member
A fundamental goal of evolutionary biology is to understand the mechanisms that maintain adaptive diversity. This dissertation focuses on the interplay of two key evolutionary mechanisms - natural selection and gene flow. While natural selection is often portrayed as a driving force of adaptive evolution, gene flow is assumed to disrupt selection by introducing maladapted alleles into locally adapted populations. Yet this paradigm is beginning to shift as a growing appreciation for the role gene flow may play in concert with natural selection to facilitate adaptative divergence. I explore this interaction of selection and gene flow in island scrub-jays (Aphelocoma insularis), a highly mobile bird experiencing local adaptation at a microgeographic scale. First, I demonstrated that observed differences in bill morphology between pine-oak ecotones are likely genetically based despite overall limited population genetic structure. Second, I found that the genetic underpinnings of divergent bill morphologies are highly parallel at higher genetic levels, which is indicative of selection acting on shared, but highly polygenic, molecular pathways. Finally, I tested alternate dispersal mechanisms potentially impacting patterns of limited gene flow and found evidence for sex-biased natal habitat preference shaping limited dispersal. Collectively, these results show how gene flow can enhance adaptive divergence at microgeographic scales.

Browsing by Author "Funk, W. Chris, advisor"

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