Alleles are the individual genes that give rise to certain characteristics in an organism.
When an allele is "fixed" to a population, this means that the characteristic will be constant & most likely will not change until the environment changes as well.
Like with arctic hare, the alleles that influence the color of their coats (white in the winter & brown in the spring/summer) will stay because any fur pattern other than the aforementioned characteristics with result in becoming someone else's food with the lack of camouflage.
Hope that helps!
