Genetic hitchhikking 

Genetic hitchhiking is the process by which an allele may increase in frequency by virtue of being linked to a gene that is positively selected. Proximity on a chromosome may allow genes to be dragged along with a selective sweep experienced by an advantageous gene nearby. More generally, genetic hitchhiking can refer to changes in an allele's frequency due to any form of selection operating upon linked genes, including background selection against deleterious mutations. Whether a neutral allele becomes fixed is a matter of chance. The traditional view of this stochastic process is that it is dominated by sampling error, that is genetic drift. But it may instead be dominated by whether the allele is linked to a good genetic background: this is known as genetic draft. (Wikipedia)