ɐ sᵻlˈɛktɪv swˈiːp kæn əkˈɜː wɛn ɐ ɹˈɛɹ ɔːɹ pɹˈiːviəsli nˈɑːnɛɡzˈɪstɪŋ ɐlˈiːl ðæt ɪŋkɹˈiːsᵻz ðə fˈɪtnəs ʌvðə kˈæɹiɚ ɹˈɛlətˌɪv tʊ ˈʌðɚ mˈɛmbɚz ʌvðə pˌɑːpjʊlˈeɪʃən ˈɪŋkɹiːsᵻz ɹˈæpɪdli ɪn fɹˈiːkwənsi dˈuː tə nˈætʃɚɹəl sᵻlˈɛkʃən.
A selective sweep can occur when a rare or previously non-existing allele that increases the fitness of the carrier (relative to other members of the population) increases rapidly in frequency due to natural selection.