Adaptation of Plants to Soils Containing Toxic Metals ‐ A Test for Conceit

Metal Tolerance

“…Most species, he believes, are in a state of genostasis, where a lack of appropriate genetic variability precludes further evolutionary change. This has been demonstrated elegantly by Gartside & McNeilly (1974) and Ingram (cited in Bradshaw, 1984) by screening for coppertolerant individuals of a range of grass species from large seedling samples derived from non-tolerant populations. There has been no case reported of a species that evolves tolerance but which does not possess variability for tolerance in its normal populations.…”

“…Adapted genotypes are preadjusted without these constraints. However, the 'costs' of tolerance may be great and manifested in terms of the reduced fitness of tolerant plants when grown in normal (uncontaminated) situations (Cook, Lefebvre & McNeilly, 1972;Cox & Hutchinson, 1981;Bradshaw, 1984). Ernst (1976) also suggests that the slower growth rates and lower biomass production of many tolerant plants by comparison with their non-tolerant counterparts are a corollary of the energy expenditure for operation of the mechanisms of tolerance involved.…”

“…This basic observation poses intriguing questions about the evolutionary processes involved in adaptation to metalliferous environments. Bradshaw (1984) concludes that it is the restriction in supply of genetic variability which directly limits the evolutionary processes and the extent of adaptation. Most species, he believes, are in a state of genostasis, where a lack of appropriate genetic variability precludes further evolutionary change.…”

“…As has been suggested earlier, these species are restricted in their micro-distribution at such chemically heterogeneous sites to weakly contaminated, more fertile soils. Their absence from the majority of mine sites probably then reflects their strict nutritional requirements and their rare occurrences at contaminated sites are instances of biological inertia, although it remains to be ascertained if these species do in fact possess tolerance to Zn, Pb and Cd in their normal populations, as has been shown to be the case for Cu (Bradshaw, 1984).…”

“…Ernst's monograph 'Schwermetallvegetation der Erde' (Ernst, 1974a) remains the standard source of reference on plants and plant communities of metal-contaminated soils on a global scale. Landmark reviews and papers on evolutionary aspects are Antonovics, Bradshaw & Turner (1971), Antonovics (1975) and Bradshaw (1976Bradshaw ( , 1984. Genetical aspects are also covered by Macnair (1981), methodology by Wilkins (1978) and mechanisms of tolerance by Turner (1969), Ernst (1974bErnst ( , 1975Ernst ( , 1976, Wainwright & Woolhouse (1975), Farago (1981, Thurman (1981), Peterson (1983) and Woolhouse (1983).…”