Soil near copper mines contains high concentrations of toxic copper ions. Most plants of the species Agrostis tenuis cannot survive in these soils. However, some individual plants possess an allele that allows them to tolerate elevated copper levels. Over several generations, the frequency of this tolerant allele increases significantly in populations growing on mine tailings.
Which of the following best describes the process leading to this evolutionary change?
Stabilising selection, because the extreme phenotype of high copper tolerance is selected against, maintaining the population's phenotype at a stable intermediate level.
Directional selection, because a change in environmental conditions favours individuals with phenotypes at one extreme of the distribution, leading to a shift in allele frequency over generations.
Artificial selection, because the selective pressure is caused by human mining activities, which constitutes deliberate selective breeding.
Genetic drift, because the increase in the frequency of the copper-tolerance allele is a result of random fluctuations in allele frequencies in a small population.