Explanation: The process that occurred in the lake is natural selection. In the presence of an external environmental influence such as pollution, only the trouts that were fit to survive or in other words those that have the alleles that help in adapting to the new conditions live and increased in numbers. In such a case, one can say the fittest of the organisms survived and the corresponding allelic frequency in that population increased over time. The organisms carrying the other alleles died due to pollution and hence the overall frequency of alleles change.
