The slag heap plants mentioned above have been examined genetically. It turned out that the plants growing on polluted ground did not acquire any new features but that within these certain species the ability of toxitolerance existed before they populated on contaminated ground. Only species that show toxitolerance on normal locations as well were able to colonize contaminated ground. Toxitolerance is part of the natural variability of these species. (The number of toxitolerant species is too high for being explainable by spontaneous mutations.)
Obviously, there was a selection of already toxitolerant forms. The exclusive (one-sided) selection leads to specialization (see pic. 69), which may be linked to a reduction of the crossbreedability with the normal form on unpolluted ground: Genetic isolation and in the end speciation take place. In this case genetic isolation can be effective for instance in a shift of flowering time.
Here the toxitolerance can probably be explained by a reduced assimilation of mineral salts out of the ground; so it does not present an evolutional improvement, but rather a reduction, that turned out to be of advantage in this specific environment of contaminated ground.
The process observed with slag heap plants shows just as similar cases that speciation cannot be seen as a beginning macroevolution, but is often connected to specialization and with this to an impoverishment of the gene pool. This is why repeated speciation rather leads to an impoverishment of the gene pool and so to a reduction of variability (see pic. 70). Although mutations can counteract the impoverishment of the gene pool, studies show that this at long sight cannot counter the trend to impoverishment. When the splitted species cannot react in a flexible way anymore to environmental changes because of their one-sided specialization, they are threatened with extinction.