The gene pool refers to all the genes in a population. Another word for gene is allele. The frequency of alleles within a population can change depending on the pressures exerted by the environment. In pre-industrial revolution England, white moths far out-numbered black moths due to the fact that white moths were able to escape predatory birds as their light color against the light colored trees better camoflauged them. After the industrial revolution, factories released soot and smoke which covered trees. The dark moths then blended in better to their surroundings and escaped the birds. Their white counterparts were eaten and the gene frequency shifted from the light allele being more prevalent, to the dark allele in just a few decades. Therefore, natural selection and the environment exerts pressure on which alleles are going to be more frequent in a population.