The evolution of plants in metal-contaminated environments

Summary

There are many areas of the world where the soil is naturally, or through anthropogenic activity, contaminated with metals. Metals are toxic to plants in excess, and the consequence has been that many species found on normal soils are excluded from these areas. The plants that can grow on these soils can normally be shown to have evolved tolerance to the metals in excess, though there are some species which may be constitutively able to tolerate high metal levels, and there is some evidence for environmentally induced tolerance in some species. Tolerance is generally under major gene control, though the degree of tolerance shown by a plant will be affected by minor genes as well, at least some of which act hypostatically to the major tolerance locus. The major tolerance loci generally are specific, so that where plants show tolerance to more than one metal, it is because they have evolved independent tolerances to more than one metal. Co-tolerance, where one gene gives pleiotropic tolerance to more than one metal, is probably rare. The mechanism of tolerance is in most cases unknown, and the problems of studying this phenomenon are discussed. There is circumstantial evidence that tolerance involves a cost, in that tolerant plants are at a selective disadvantage in an uncontaminated environment. It has not, however, been possible to establish the reason or basis of this cost. The ability of a species to evolve tolerance seems to depend on the presence of tolerance genes at low frequency in normal populations prior to the selective agent being imposed. In areas which have been naturally contaminated for very long periods, endemic species restricted to the toxic environment are found (edaphic endemics). The evolutionary processes leading to these, and the difference between an endemic and an ecotype, are discussed.