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- Experimental evolution and ecological consequences : new niches and changing stoichiometry
- Turner, Caroline B.
- Electronic Theses & Dissertations
Evolutionary change can alter the ecological conditions in which organisms live and continue to evolve. My dissertation research used experimental evolution to study two aspects of evolutionary change with ecological consequences: the generation of new ecological niches and evolution of the elemental composition of biomass. I worked with the long-term evolution experiment (LTEE), which is an ongoing experiment in which E. coli have evolved under laboratory conditions for more than 60,000...
Show moreEvolutionary change can alter the ecological conditions in which organisms live and continue to evolve. My dissertation research used experimental evolution to study two aspects of evolutionary change with ecological consequences: the generation of new ecological niches and evolution of the elemental composition of biomass. I worked with the long-term evolution experiment (LTEE), which is an ongoing experiment in which E. coli have evolved under laboratory conditions for more than 60,000 generations. The LTEE began with extremely simple ecological conditions. Twelve populations were founded from a single bacterial genotype and growth was limited by glucose availability. In Chapter 1, I focused on a population within the LTEE in which some of the bacteria evolved the ability to consume a novel resource, citrate. Citrate was present in the growth media throughout the experiment, but E. coli is normally unable to consume it under aerobic conditions. The citrate consumers (Cit+) coexisted with a clade of bacteria which were unable to consume citrate (Cit-). Specialization on glucose, the standard carbon source in the LTEE, was insufficient to explain the frequency-dependent coexistence of Cit- with Cit+. Instead Cit– evolved to cross-feed on molecules released by Cit+. The evolutionary innovation of citrate consumption led to a more complex ecosystem in which two co-existing ecotypes made use of five different carbon sources.After 10,000 generations of coexistence, Cit- went extinct from the population (Chapter 2). I conducted replay experiments, re-evolving for 500 generations 20 replicate populations from prior to extinction. Cit- was retained in all populations, indicating that the extinction was not deterministic. Furthermore, when I added small numbers of Cit- to the population after extinction, Cit- was able to reinvade. It therefore appears that the Cit- extinction was not due to exclusion by Cit+, but rather to unknown laboratory variation.Chapter 3 shifts focus to studying evolutionary changes in stoichiometry, the ratio of different elements within organisms’ biomass. Variation in stoichiometry between organisms has important ecological consequences, but the evolutionary origin of that variation had not previously been studied experimentally. Growth in the LTEE is carbon limited and nitrogen and phosphorus are abundant. Additionally, daily transfer to fresh media selects for increased growth rate, which other research has suggested correlates to higher phosphorus content. Consistent with our predictions based on this environment, clones isolated after 50,000 generations of evolution had significantly higher nitrogen and phosphorus content than ancestral clones. There was no change in the proportion of carbon in biomass, but the total amount of carbon retained in biomass increased, indicating that the bacteria also evolved higher carbon use efficiency.To test whether the increases in nitrogen and phosphorus observed in the LTEE were a result of carbon limitation or were side effects of other selective factors in the experiment, I evolved clones from the LTEE for 1000 generations under nitrogen rather than carbon limitation (Chapter 4). The stoichiometry of the bacteria did change over the course of 1000 generations, indicating that evolution of stoichiometry can occur over relatively short time frames. Unexpectedly however, the evolved bacteria had higher nitrogen and phosphorus content. It appears that the bacteria were initially poor at incorporating nitrogen into biomass, but evolved improved nitrogen uptake.
- Preventing predation : evolution and adaptive plasticity in morphological defense of an invasive species
- Miehls, Andrea Lynn-Jaeger
- Electronic Theses & Dissertations
Invasive species are one of the leading threats to global biodiversity, but until recently, little attention has been paid to how evolution contributes to invasive species effects despite important applications to conservation and our general understanding of evolution in the wild. Invasive species often experience strong selection upon introduction, which may result in adaptive evolution that could facilitate their successful integration into food webs and their effects on native species....
Show moreInvasive species are one of the leading threats to global biodiversity, but until recently, little attention has been paid to how evolution contributes to invasive species effects despite important applications to conservation and our general understanding of evolution in the wild. Invasive species often experience strong selection upon introduction, which may result in adaptive evolution that could facilitate their successful integration into food webs and their effects on native species. Recent species invasions in North America offer a strong opportunity to address fundamental questions in evolutionary ecology as well as advance our understanding of invasive species effects.
Bythotrephes longimanus(the spiny water flea) is a predatory zooplankton with a conspicuous tail spine that invaded the Great Lakes region during the 1980s and may be having large negative effects on fisheries. Previous field studies show the morphology and life history of Bythotrephesstrongly vary spatially and temporally, but the cause is not known. Evolution by natural selection and phenotypic plasticity are potential sources of Bythotrephestrait variation, but heretofore, these sources of variation have not been investigated.My dissertation research investigated ecological and evolutionary factors that influence morphological and life history variation in Bythotrephes. Using Bythotrephescollected from Lake Michigan, I found moderate-to-high genetic variation in distal spine and body length and maternal effects in both traits. Further, experiments revealed that spine length, body size, and clutch size respond plastically to temperature but not to fish predator cues, with higher temperature inducing mothers to have smaller clutches of larger offspring (longer absolute distal spine and body length) that were better defended against predation. Although Bythotrephesuse temperature as the proximate cue of plasticity, it is likely that the trait changes represent adaptations to varying fish predation risk which correlates with water temperature. I also found temporally and spatially variable selection on distal spine length consistent with seasonal changes in gape-limitation of fish predators and spatial heterogeneity of fish, respectively. Yet, despite net selection for increased distal spine length, I observed little evidence of an evolutionary response to selection based on comparisons of historic and contemporary wild-captured individuals and retrieved spines from sediment cores. In a companion study of Canadian Shield lakes, I identified gape-limited fish predators as agents of selection on Bythotrephesdistal spine length. Specifically, I found selection for increased distal spine length in lakes dominated by a gape-limited fish predator and no significant selection in lakes dominated by a non-gape-limited fish predator. A large difference (20%) in average distal spine length between lakes of each predator type was consistent with the direction of selection, suggesting potential local adaptation of distal spine length to gape-limited fish predation.The results of my dissertation indicate Bythotrephesrespond to fish predation through multiple mechanisms, including phenotypic plasticity and evolutionary responses to selection. These responses to predation likely promote Bythotrephessuccess as an invasive species, and may also underlie negative effects on important Great Lakes fisheries through food web interactions. More generally, the results of my dissertation suggest the effects of invasive species may occur not just through their ecological interactions, but also through evolutionary and phenotypically plastic trait modifications. As invasive species continue to affect biodiversity worldwide, understanding the mechanisms behind invasive species effects is critical.