A population of butterflies is highly polymorphic with different genes encoding for color on different parts of their wings.
The allele encoding for a black hindwing is rare in the population. However, just by chance the butterflies with red forewings also have black hindwings.
Quite suddenly, selection comes to strongly favor butterflies with red forewings, causing red tipped butterflies to become more prevalent. Thus the proportion of black hind wings also increases, even though it does not confer any survival benefit.
This illustrates the principle of Hitch-hiking: black alleles increase their frequency simply because they are linked to red alleles.