Evolutionary ecology of plant adaptation to serpentine soils

Plant adaptation to serpentine soil has been a topic of study for many decades, yet investigation of the genetic component of this adaptation has only recently begun. We review the defining properties of serpentine soil and the pioneering work leading to three established physiological and evolutionary mechanisms hypothesized to be responsible for serpentine tolerance: tolerance of a low calcium-to-magnesium ratio, avoidance of Mg toxicity, or a high Mg requirement. In addition, we review recent work in serpentine ecology documenting the high proportion of endemic species present, the adaptive morphologies of serpentine-tolerant plants, and the distinctive structure of serpentine communities. Studies of the physiological mechanisms proposed to confer serpentine tolerance have shown that uptake of particular ions and heavy metals varies between serpentine-tolerant and -intolerant species. Recent studies examining the genetic basis of serpentine adaptation have shown serpentine-adaptive quantitative trait loci (QTL) to have large phenotypic effects, drought tolerance to be as important as metal tolerance, and serpentine adaptation to have evolved independently multiple times within species. Investigations of plant races and species adapted to contrasting soil types have shown disparate flowering times, divergent floral morphologies, and pollen incompatibility to contribute to reproductive isolation. Finally, we propose that future studies involving serpentine systems should merge the fields of ecology, evolution, physiology, and genetics.