Fixation of a neutral allele within the population is proportional to its frequency in the population. So, even something that is initially useless can linger i

Fixation of a neutral allele within the population is proportional to its frequency in the population. So, even something that is initially useless can linger in the population, and even become fixed within the population. If that neutral allele is lingering in the population at a low frequency and suddenly becomes advantageous, there will subsequently be strong signals of selection at that locus.