Browsing by Subject "experimental evolution"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Causes and consequences of evolutionary innovation: An experimental approach to evaluating assumptions and predictions in macroevolutionary theory
    (2020-01) Gettle, Noah
    It has long been noted that there are some adaptations that appear to have played a disproportionate role in determining the evolutionary trajectories of the clades in which they arose. These adaptations, often termed evolutionary innovations, are often associated with increases in diversity and expansions into new niche spaces. The historic nature of evolutionary innovations, however, largely limit our ability to draw conclusions about causes and consequences, leaving broad-scale explanations constrained to theory. Using the power of experimental evolution, this work aims to explore empircally theories concerning the origins and evolutionary consequences of innovations. I used one proposed innovation, multicellularity, a trait that reliably arises in brewers’ yeast (Saccharomyces cerevisiae) under certain selective conditions. Using genomic tools, I show that despite their disruptive nature, loss-of-function mutations in largely “non-regulatory genes” are the major causal genetic changes underpinning convergent evolution of experimental yeast populations toward multicellularity. I further show that one of these mutations is also associated with major transcriptional and physiological effects one of which, increased apoptosis, has been previously described as a multicellular adaptation. Data presented here suggests this is less likely a direct effect of loss of gene activity than of microenvironmental shifts associated with a multicellular lifestyle. Finally, I present research that suggests that adaptive responses to environmental challenges often associated with complex multicellularity, such as division of labor, may not represent optimal fitness solutions but rather reflect a balance between the costs and benefits of retained multicellularity. In sum, my results reveal that current theories regarding multicellularity as well as other innovations may, at best, be incomplete and that generalizations about causes and consequences of evolutionary innovations may prove more difficult to come by than many have suggested.
  • Thumbnail Image
    Item
    Genotypic and Phenotypic Evolution in Experimental Microbial Populations: Causes and consequences of an evolutionary reversal across a major transition
    (2022-05) Khey, Joleen
    Multicellularity is an evolutionary transition which opened up new avenues for adaptation that were inaccessible to unicellular life forms. In this dissertation, I outline one of the few sets of experiments where the effect of history, chance, and adaptation have been studied across a major evolutionary transition – the evolution of multicellularity. I carry out experimental evolution studies to elucidate the extent to which history can feedback and influence future evolutionary trajectories. It has been shown that unicellular yeast can evolve multicellularity by selection for rapidly falling to the bottom of a test tube (“settling selection”). Previously, yeast lines were selected for size, resulting in multicellularity and then selected on agar plates, resulting in reversal to unicellularity. The three experiments described in this dissertation start with these secondarily unicellular yeast strains. Using the same selection scheme described above, I select for reversion to multicellularity. In general, there is quicker reversion to multicellularity in the second round of evolution compared to their naïve unicellular ancestor. There is also strong parallelism in the tempo of reversion among replicate populations, but not between lineages. The genetic basis for the reversion to multicellularity was also evaluated. Differences in genetic basis for the reversion to multicellularity compared to the initial selection experiment demonstrate the importance of historical contingency on the genotypic level. In this first round of settling selection, multicellularity is a single-locus trait, however, after the second round, multicellularity was polygenic. Finally, I examine a surprising consequence of history – the emergence of phenotypic plasticity in a secondarily unicellular isolate. This is one of the few experimental evolution studies on phenotypic plasticity. I show that there is a trade-off associated with the plastic phenotype. Extended experimental evolution to select for further plasticity yielded minimal improvements suggesting that there are evolutionary constraints to the evolution of this phenotype. This research allows us to gain a better understanding of how previous historical events can influence evolution and the predictability of evolution at both the phenotypic and genotypic levels. Historical contingency has far-reaching phenotypic and genotypic consequences, which add to the complexity inherent in biology. This is evident in a system as simple (or as complex) as laboratory yeast subject to falling to the bottom of a test tube.
  • Thumbnail Image
    Item
    The hidden costs of rapid adaptation: experimentally assessing the effects of standing variation on the pace and trajectory of evolution
    (2022-03) Griffin, Josie
    As the planet changes at an alarming rate, there is a great need to understand why some populations are better equipped to rapidly adapt to their new environment than others. Many factors contribute, but populations are ultimately limited in their pace by their genetic makeup—they either have variants that allow them to survive or they do not. But, in the race to adapt, all sources of variation are not equal, and standing genetic variation is theorized to be of the most benefit in contributing to rapid adaptation. Here I explore the role of standing variation, both in a population’s ability to adapt at a rapid pace and in the potential long term evolutionary consequences that occur as a result. My work confirms expectations that increased standing variation in a population allows for a faster rate of adaptation, but although these populations are able to succeed in the short term, but this achievement comes at a significant cost to long term viability. All populations, across all experiments, that utilize standing variation as the genetic basis for rapid adaptation lose the ability to undergo sexual recombination, and therefore lose an important mechanism for maintaining variation in the long term. I begin by determining how the amount of standing variation present in a population correlates to the timing and rate of a successful adaptive response to a stressful environment. I assess how this result is intertwined with loss of sex and explore the mechanism for that loss. Then, I explore how the dynamics of the system change if the environmental shift occurs gradually rather than as a dramatic climactic event. Finally, I compare the variety of evolutionary strategies that develop in populations that began with standing variation versus mutation as their genetic substrate and evaluate their potential for success in the long term. Taken together, these results present a different picture of the role of standing variation than might be assumed. It does indeed allow for rapid adaptation, but the increased degree of genetic variation is not an evolutionary panacea and may send populations down evolutionary trajectories that are short-sighted.
  • Thumbnail Image
    Item
    Investigating collective action as a scaffold for eco-evolutionary feedbacks
    (2021-12) Wang, Pu
    Eco-evolutionary feedbacks involve situations where environmental conditions influence evolutionary changes, which, in turn, feedback to the environment. Such interactions between ecological and evolutionary processes are prevalent in many biological systems from phage and bacteria to forests and animals. An increasing number of research projects address the significant impact of eco-evolutionary feedbacks in shaping the diversity of living organisms and their living niche, in many ecosystems in the modern world. Furthermore, interactions between ecology and evolution are taking place all the time. Thus, eco-evolutionary feedbacks would have significantly impacted the evolutionary history of life. The evolution of collective action also has significantly changed the form of life, by changing the way selection worked on organisms. Multicellularity, one example of the collective action of cells, is considered as one of the major transitions of life. The significance of such a major transition has been addressed more from promoting new organization or functions. However, the other, even more impactful, aspect of influencing the entire community via eco-evolutionary feedbacks has been less emphasized. Populations experiencing major transitions could alter the environments innovatively that would feedback on the subsequent evolutionary changes. In my thesis research, I addressed this impactful but less emphasized aspect of a major transition’s significance, by investigating the effect of eco-evolutionary feedbacks via collective actions. My projects demonstrated collective action is a scaffold for eco-evolutionary feedbacks from three perspectives. Firstly, I documented the evolutionary process of collective action. Then I created a predator-prey microcosm to describe how eco-evolutionary interactions occur along with the emergence of collective action. Lastly, I used genome sequencing techniques to investigate how eco-evolutionary interactions influence the entire communities including unevolved ones. I took the experimental evolution approach with different microbial systems to address each perspective. My research provided an empirical basis for investigating the important, yet less addressed aspect of the impact of major transitions. The effect of major transitions affected the evolving organism itself as well as the rest of the community by shaping the environment with new structures and functions. It also provides insights into the impact of eco-evolutionary feedbacks in the history of life.