Speciation rates vary tremendously among lineages, and our understanding of whatfuels the rapid succession of speciation events within young adaptive radiations remains incomplete [1][2][3][4][5][6][7][8][9][10][11] . The cichlid fish family provides a notable example of such variation, with many slowly speciating lineages as well as several exceptionally large and rapid radiations 12 . By reconstructing a large phylogeny of all described cichlid species, we show that explosive speciation is solely concentrated in several large young lake species flocks. Speciation rate increases are associated with absence of top predators but this is not a sufficient explanation for explosive speciation. Across lake radiations we observe a positive relationship between speciation rate and enrichment with large indel polymorphisms. Assembly of one hundred cichlid genomes within the most rapidly speciating cichlid radiation, found in Lake Victoria, reveals exceptional 'genomic potential' -hundreds of ancient haplotypes bearing indel polymorphisms, many associated with specific ecologies and shared with ecologically similar species from other older radiations elsewhere in Africa. Network analysis reveals fundamentally nontreelike evolution through recombining old haplotypes, with origins of ecological guilds concentrated early in the radiation. Our results suggest that the combination of ecological opportunity, sexual selection and exceptional genomic potential is the key to understanding explosive adaptive radiation.

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