A population of wolves migrates north in search of food. as the wolves travel, they enter a region that has long, snowy winters. some of the wolves have an allele that gives them white coats. this makes them better able to sneak up on prey.

This is the case of selective pressure of environmental factors like snow, due to which favored the expression of the allele that formed white coat phenotype. This is not a Hardy-Weinberg principle, in which no selective pressure is applied. The relative frequencies of the alleles change in the gene pool, due to selective pressure, and the expressed alleles are often significant for survival in those conditions.