The adaptive radiation of haplochromine cichlid fishes in Africa's Lake Victoria region — more than 700 species arising within roughly the last 150,000 years — was shown by Meier et al. (2017) to have been seeded by hybridization between two divergent lineages, whose admixture resuspended old genetic variation that was recombined and sorted into hundreds of new species. Two causality questions raised by that finding remain open. First, at the locus level: no introgressed allele in this radiation (including the admixture-derived LWSLWS opsin variants and putative Bateson–Dobzhansky–Muller incompatibility loci) has yet been functionally demonstrated to causally affect reproductive isolation or ecological performance. Second, at the macroevolutionary level: whether ancient hybridization is a common causal factor promoting exceptionally rapid, species-rich adaptive radiations across taxa — as distinct from correlated phenomena such as genome duplication or ecological opportunity alone — is unresolved, supported by accumulating single-system case studies but contested by comparative analyses.

Background

An adaptive radiation is the rapid diversification of a single ancestral lineage into many species that exploit different ecological niches. Explaining why some lineages radiate explosively while others do not — speciation rates vary by orders of magnitude across the tree of life — is a central problem in evolutionary biology. Haplochromine cichlid fishes of the Lake Victoria region (the 'Lake Victoria Region Superflock', more than 700 species in Lake Victoria and neighboring lakes) constitute one of the fastest known vertebrate radiations, with all species arising well within the last 150,000 years. Hybridization is interbreeding between distinct lineages; admixture and introgression denote the resulting incorporation of genetic material from one lineage into another. A hybrid swarm is a population of mixed, recombinant ancestry formed when previously diverged lineages meet in secondary contact.

Classically, hybridization was viewed mostly as a homogenizing force that erodes species boundaries, although Seehausen (2004) had proposed that it could also promote adaptive radiation by injecting variation. Meier, Marques, Mwaiko, Wagner, Excoffier and Seehausen (2017) provided genomic evidence that this is what happened in the Lake Victoria Region Superflock: two divergent riverine haplochromine lineages hybridized, and the resulting hybrid swarm's variation was recombined and sorted into hundreds of new species. Two observations were central. First, the hybridization event generated exceptional allelic variation at the long-wavelength-sensitive (LWSLWS) opsin gene — a visual pigment gene previously implicated in cichlid adaptation and speciation to different light environments. Second, genetic differentiation between the new species is accentuated around variants that were fixed differences between the ancestral parental lineages, now reshuffled into new combinations across species (a signature of 'combinatorial' speciation from ancient admixture-derived standing genetic variation, i.e., variation already present in the founding population rather than arising by new mutation). This made hybridization a candidate general explanation for rapid radiation. The authors noted unresolved questions at two levels: in the Discussion, whether hybridization in the ancestry of major adaptive radiations is widespread, and whether its occurrence may explain some of the observed large variation in the rates and volume of species diversification among lineages; and in the Introduction, that a direct link between introgressed traits and speciation awaits further testing — a question that concerns specific introgressed alleles such as the admixture-derived LWSLWS variants and putative Bateson–Dobzhansky–Muller (BDM) incompatibility loci (i.e., negatively interacting gene combinations from different parental genomes that reduce hybrid fitness) — and that where hybridization coincides with polyploidy (whole-genome duplication, common in plant radiations), effects of hybridization per se are difficult to distinguish from those of gene or genome duplication. Distinguishing true introgression from incomplete lineage sorting (ancestral polymorphism retained by chance across speciation events) and from unsampled 'ghost' lineages is a general methodological difficulty in this literature.

Problem Statement

The problem, as raised by Meier et al. (2017) and still open, has two causally linked dimensions — establishing (or refuting) ancient admixture as a causal motor of adaptive radiation, at the locus level and at the macroevolutionary level.

(i) Locus-level causality in the hybrid-swarm-derived cichlid radiation: identify specific introgressed loci and alleles of the Lake Victoria region radiation — including the admixture-derived LWSLWS opsin variants, putative Bateson–Dobzhansky–Muller incompatibility loci, and the oligogenic trait modules of hybrid origin detected by genotype–phenotype analyses — and determine whether and how these variants causally affect reproductive isolation or ecological performance, establishing the direct link between introgressed traits and speciation that the source notes awaits further testing. Functional approaches to such a test that are becoming available in this system include, for example, allele replacement or genome editing, controlled crosses with fitness and mating assays, and ecological-performance assays of defined genotype classes.

(ii) Macroevolutionary generality and causal attribution: determine whether ancient hybridization of the type documented in the Lake Victoria region (a hybrid swarm formed by secondary contact of divergent lineages) is widespread in the ancestry of major adaptive radiations across taxa and regions, and whether its occurrence helps explain the large variation in the rates and volume of species diversification among lineages — as opposed to being a correlate of radiations that arise chiefly from ecological opportunity, genome duplication, or other factors.

The two questions bear jointly on how broadly the combinatorial-hybridization mechanism explains the orders-of-magnitude variation in species diversification rates: (i) would supply the mechanistic proof of principle inside the best-documented case, and (ii) would test whether that mechanism generalizes. Not part of the question: the timing of the radiation relative to lake history and the rough demography of the hybrid swarm, which subsequent whole-genome dating has substantially resolved (all Lake Victoria trophic guilds diverged within roughly the last 16,700 years, with parallel lakes similarly young), though fine-scale demographic reconstruction of the founding swarm continues.

Completion standard: the problem is fully resolved only when all four mandates are addressed, and each is independently refutable — (i-a) identify specific introgressed loci and alleles across the LWSLWS, BDM-incompatibility and oligogenic-module classes; (i-b) functionally determine whether such a variant causally affects reproductive isolation, or, for an ecological variant, ecological performance (either axis establishing the direct link to speciation); (ii-a) establish whether hybrid-swarm ancestry is widespread across major radiations; (ii-b) test whether that ancestry explains diversification-rate variation net of ecological opportunity, genome duplication, and other correlates. Settling only part of a dimension, or returning a negative functional result for a given locus, resolves that part only and leaves the remaining mandates open.

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