Phenotypic diversity underlies life as we know it-a variety of species, each with different roles, are essential for a fully functioning ecosystem just as a range of different crops is necessary to provide different nutritional value. Even within a single species, individual variation promotes diversity, allowing for adaptation to new environments and achievements despite challenges. Characterizing this phenotypic diversity and understanding the factors that drive it is a principal goal of

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... gical research. The fruits of this research could have the power to transform the way farmers cultivate agriculture and breed livestock, the way scientists discover cures, and the way doctors predict, treat, and prevent disease. Scientists have long understood that genetic variation is a main contributor to phenotypic variation and recent advancements in sequencing technologies have improved characterization of genetic variation among species. However, high environmental variation interferes with scientists' abilities to connect phenotypic variation, such as patient response to a chemotherapeutic drug, to a single genetic variant. For this reason, model organisms such as the roundworm nematode Caenorhabditis elegans have become forerunners in the field of quantitative genetics. In this dissertation, I investigate how natural genetic variation in C. elegans shapes the diverse phenotypes observed across the species. While I mostly focus on the nematode response to a variety of toxins including chemotherapeutics, anthelmintics, and heavy metals, I also explore how genetic variation might play a role in the species' adaptation to new climates. This work highlights the power of our system to connect phenotypic variation to causal genetic variants. Furthermore, this work aims to answer several open questions in the field of quantitative genetics such as understanding the molecular mechanism of phenotypic trait evolution and determining the extent of genetic complexity of phenotypic traits.