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Abstract
Hybridization is an important evolutionary process leading to the establishment of new species. Ecological speciation, or the formation of new species by colonization of novel niche, is a process by which newly-formed hybrids may persist alongside parental lineages. Helianthus annuus and H. petiolaris hybridized multiple times, giving rise to three hybrid species, which currently occupy different habitats. One of these hybrid species, H. anomalus, grows on sand dunes in the western United States, which have much lower fertility than the soils where the parental species are found. We employ a comparative approach within this known phylogenetic context to explore the role of nutrient limitation in the evolution of H. anomalus. We find that H. anomalus is more tolerant of nutrient stress than H. annuus and H. petiolaris. This tolerance appears to be conferred by increased tissue longevity that allows nutrients to be used more efficiently but comes at the cost of a reduced maximum relative growth rate. We present evidence that stress response genes, including one that is likely to affect leaf turnover rates, respond differently to nutrient stress in H. anomalus compared to its parental species. We find evidence of selection on several physiological traits important in plant water and nitrogen use strategies, suggesting that water and nutrients have been important forces in the evolution of H.anomalus. Finally, we detect a pair of large-effect quantitative trait loci that affect foliar nitrogen concentrations, which may have been important in the rapid evolution of H. anomalus.