Edaphic endemism

One of the features of areas of the world that have had naturally occurring toxic soils for many thousands or millions of years is that these soils often support species that are found nowhere else so-called edaphic endemics (Kruckeberg and Rabinowitz, 1985). Examples of such soils include the Great Dyke in Zimbabwe and serpentine soils in many areas of the world. It has been argued on many occasions (eg, Proctor and Woodell, 1975 Kruckeberg, 1984) that the phenomenon of metal tolerance provides a model of how these species may have started on the evolutionary route that finally led to full edaphic endemism. 

The evolution of the tolerant ecotype is conceptually easy. As described above, first tolerance evolves, and then further adaptations to the other features of the mine environment can be expected to spread. Other genes that enhance the tolerance produced by the initial gene(s) (modifiers) increase the adaptation (Macnair, 1983 Schat and ten Bookum, 1992 a). The problem is, how does an ecotype evolve into a full species Kruckeberg (1984) points out that on the California serpentines there are about 250 endemic taxa, but a further 1000 species that are found both on and off the serpentine (bodenvag species), most of which can be expected to have evolved serpentine-tolerant taxa. What is the difference between a bodenvag species and a species that has evolved into an edaphic endemic ... 

Heavy metal tolerance remains one of the clearest examples of microevolution, and one of the best systems to study the relationship between adaptation and ecology. It provides a model for the evolution of ecotypes and edaphic endemics. At the physiological and biochemical level, it is a model system for the study of the mechanisms of resistance to stress and pollution. Many questions remain unanswered, however, among the principal ones of which are ... 

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