The necessity of genetic diversity for evolution to occur is illustrated by two plant examples. Plants with genetic diversity for ability to tolerate heavy metal pollution (Agrostis tennuis and bunch grass) were able to colonize the polluted soils (containing Cu, Zn and Cd) on slagheaps from mines in Wales, UK. They achieved this by evolving heavy-metaltolerant forms. Conversely, those plant species without the appropriate genetic diversity failed (Bradshaw 1991). Similarly, the American chestnut was almost driven to extinction by an introduced disease to which it had no genetic diversity for resistance.