Thale cress (Arabidopsis thaliana) is a small, quickly reproducing, edible plant. It has a gene locus with an allele that results in a tolerance to heavy metals, which can leech into the soil as a result of mining operations.
A genetics professor uses a population of thale cress for a spring semester class to practice genetic analysis skills. The professor continues using the population after mining started near the collection location and tracks their data from year to year. The graph displays the data for the allele frequencies for a heavy metal tolerance gene.
Based on the evidence, during what year was the mine built?