The coincidence of ecological opportunity with hybridization explains rapid adaptive radiation in Lake Mweru cichlid fishes

Abstract

The process of adaptive radiation was classically hypothesized to require isolation of a lineage from its source (no gene flow) and from related species (no competition). Alternatively, hybridization between species may generate genetic variation that facilitates adaptive radiation. Here we study haplochromine cichlid assemblages in two African Great Lakes to test these hypotheses. Greater biotic isolation (fewer lineages) predicts fewer constraints by competition and hence more ecological opportunity in Lake Bangweulu, whereas opportunity for hybridization predicts increased genetic potential in Lake Mweru. In Lake Bangweulu, we find no evidence for hybridization but also no adaptive radiation. We show that the Bangweulu lineages also colonized Lake Mweru, where they hybridized with Congolese lineages and then underwent multiple adaptive radiations that are strikingly complementary in ecology and morphology. Our data suggest that the presence of several related lineages does not necessarily prevent adaptive radiation, although it constrains the trajectories of morphological diversification. It might instead facilitate adaptive radiation when hybridization generates genetic variation, without which radiation may start much later, progress more slowly or never occur.

Hybridizing salamanders experience accelerated diversification

“…Perhaps this combination of the primarily sessile nature of many salamander species and frequent repeated secondary contact leads to hybridization occurring regularly across evolutionary timescales. Under these scenarios, hybridization may then play a creative evolutionary role in the diversification process similar to that observed in haplochromine cichlids 24,25,77 . Allopatric speciation of haplochromine cichlids has occurred in lakes that frequently have dried, split, and reformed, whereas sympatric speciation has occurred within lakes in which lineages exhibited extreme habitat specificity and have been reproductive isolated at fine spatial scales 78 .…”

Triad hybridization via a conduit species

“…For example, we would expect it in species-rich adaptive radiations that have diversified relatively recently. Foremost among them are the hundreds of cichlid fish species in the Great Lakes of Africa, for which genome data are becoming rapidly available (78)(79)(80)(81)(82)(83)(84), and the many species of Heliconius butterflies that participate in mimicry rings (3,26,43,85). Other possible groups are numerous species complexes that are similar morphologically and genetically and sometimes difficult to resolve taxonomically, such as Cottoid fish in Lake Baikal (86), Anastrepha flies (87,88), ant-nest beetles (89), some groups of mosquitoes (44) and, among plants, Andean lupins (90) and the Hawaiian Silversword Alliance (91).…”

High-quality chromosome-level genomes of two tilapia species reveal their evolution of repeat sequences and sex chromosomes

“…The great diversity of phenotypes and sex chromosomes generated in a relatively short evolutionary time range makes African cichlids a classic model for studying the mechanisms of species radiation and sex chromosome transitions. Since the release of five representative cichlid genomes over five years ago [3], analyses of more cichlids’ (e.g., those from Lake Malawi [56, 57] and Lake Mweru [58]) and higher qualities of genomes (e.g, that of ONEg [22]) have been published, demonstrating the lasting interest in these species. Here we focused on two important aquaculture species from the much less species-rich but much more widely distributed Oreochromis genera that have undergone very recent transition between XY and ZW sex chromosome systems.…”