Plants adapted to serpentine and other ‘harsh’ edaphic (soil) conditions are ideal for investigating evolutionary processes, including intraspecific variation, adaptation, reproductive isolation, ecotypic differentiation, and the tempo and mode of speciation. Common garden and reciprocal transplant approaches show that both local adaptation and phenotypic plasticity contribute to edaphic specialization. Edaphic specialists evolve rapidly and repeatedly in some lineages, offering opportunities to investigate parallel evolution, a process less commonly documented in plants than in animals. Edaphic adaptations are often under the control of major genes and they frequently have direct or indirect effects on genes that contribute to reproductive isolation. Both reduced competitiveness and greater susceptibility to herbivory have been documented among some edaphic specialists when grown in ‘normal’ soils, suggesting that a high physiological cost to tolerance may result in strong divergent selection across edaphic boundaries. Interactions with microbes, herbivores, and pollinators influence edaphic specialization either by directly enhancing tolerance to edaphic extremes or by reducing gene flow between divergent populations. Climate change may further restrict the distribution of edaphic specialists due to increased competition from other taxa or, expand their ranges, if preadaptations to drought or other abiotic stressors render them more competitive under a novel climate.