Genecological investigations on zinc plants II. Introgression in a small population of the zinc violet Viola calaminaria ssp. westfalica (Lej.) Ernst

The two main populations of Viola calaminaria ssp. westfalica (on a mine and a neighbouring meadow) have been compared. In both, the variability is considerable, but it is greater in the mine population, where hybrids with V. arvensis occur much more frequently. Although the gene flow in the mine population must be considerable, the two populations have not diverged very much and there is no overlap in morphological characters between V. calaminaria ssp. westfalica and V. arvensis. The latter can be explained by assuming that selection pressure for heavy metal tolerance is very strong. The former phenomenon is more complicated. The isolation of the two populations is probably of recent origin, so they could not have diverged very much. If this were the case, an equilibrium between gene flow and selection could explain the remarkable variation in these small populations.