The diagram illustrates a population of the grass Agrostis capillaris growing across a boundary between an abandoned copper mine spoil heap (highly toxic soil) and normal pasture land.
Despite continuous gene flow via wind-blown pollen dispersal between the two zones, the plants on the spoil heap are highly copper-tolerant, whereas pasture plants are non-tolerant.
Which of the following statements correctly identifies the type of selection acting on the mine spoil heap population and its direct evolutionary effect?
Stabilising selection, which reduces genetic variation on the spoil heap by selecting against extreme phenotypes.
Directional selection, which increases the frequency of the copper-tolerance allele on the spoil heap by selecting against non-tolerant individuals.
Disruptive selection, which maintains both tolerant and non-tolerant phenotypes in equal proportions within the spoil heap population.
Artificial selection, because human mining activity created the selective pressure, leading to a deliberate breeding programme by farmers.