Factors ranging from ecological opportunity to genome composition might explain why only some lineages form adaptive radiations. While being rare, particular systems can provide natural experiments within an identical ecological setting where the factors promoting increased species numbers and phenotypic divergence in two closely related lineages is notably different. We investigated one such natural experiment using two de novo assembled and 40 re-sequenced genomes and asked why two closely related Neotropical cichlid fish lineages, the Amphilophus citrinellus species complex (Midas cichlids; radiating) and Archocentrus centrarchus (Flyer cichlid; non-radiating), have resulted in such disparate evolutionary outcomes. Although both lineages inhabit many of the same Nicaraguan lakes, whole-genome inferred demography suggests that priority effects are not likely to be the only cause of the dissimilarities. Also, genome-wide levels of selection, transposable element dynamics, gene family expansion, major chromosomal rearrangements, and the number of genes under positive selection were not markedly different between the two lineages. To more finely investigate particular subsets of the genome that have undergone adaptive divergence in Midas cichlids, we also examined if there was evidence for molecular pre-adaptation in regions identified by QTL mapping of repeatedly diverging adaptive traits. Although most of our analyses failed to pinpoint substantial genomic differences, we did identify functional categories containing many genes under positive selection that provide candidates for future studies on the propensity of Midas cichlids to radiate. Our results point to a disproportionate role of local, rather than genome-wide factors underlying the propensity for these cichlid fishes to adaptively radiate.

The method for the genomes and transcriptomes assemblies was described in the paper "The Comparative Genomic Landscape of Adaptive Radiation in Crater Lake Cichlid Fishes".