* assessed by mtDNA only.
The cases of divergence between sympatric ecomorphs in various rivers are ranked from 1 to 3 points. The results can be treated as a row of situations from ecologically non-functional polymorphism (1 point)via ecologically functional polymorphism (2 points) to divergent evolution or completed speciation (3 points in the presence of genetic divergence). The case of the Sore and Gojeb (2 points) showing the morphological and trophic divergence but no genome-wide divergence can be considered an initial stage of speciation with only few loci involved. Studies on genomic differentiation of thick-lipped phenotype in Amphilophus cichlids from Nicaraguan lakes showed that only a small set of loci are responsible for this phenotype (Kautt et al., 2012; Machcado-Schiaffino et al., 2017; Kautt et al., 2020; Sowersby et al., 2021). As for a sequence of divergence events, the evolution of thick-lipped species of Labeobarbus might be realized into three steps: i) phenotypic divergence, ii) ecological (trophic) divergence, iii) genetic divergence. At the last stage when all three types of divergence (phenotypic, ecological, and genetic) begin to be detectable a species status is achieved. Accordingly, the sympatric pair from the Didessa River can be considered two different species. Notable divergence in population genomic structure based on ddRAD SNPs in the lack of sorting in mtDNA evidences for a rather recent speciation in the case of the Didessa River radiation.
We detected a gradual increase of divergence among the population pairs evolving in parallel. Similar situation of the sympatric differentiation is reported for the cichlid genus Amphilophus which demonstrates a speciation continuum from incipient to fully completed (Torres-Dowdall & Meyer, 2021; Sowersby et al., 2021). These examples correspond to the concept of speciation continuum that is considered a speciation as a continuum of stages of reproductive (and other) isolations (Drès & Mallet, 2002; Stankowski & Ravinet, 2021; Bolnick et al., 2023). It is noteworthy that such a continuum is found in Labeobarbuspopulations inhabiting the rivers while all previous examples (among fishes) came from the lacustrine environment. Generally, ecological speciation is more common among fishes under the lacustrine conditions (reviewed in Seehausen & Wagner, 2014). However, the Ethiopian cyprinids (Levin et al., 2019; 2020; 2021a,b; Golubtsov et al., 2021) and South American cichlids (Burress et al., 2018), as well as the Kamchatka salmonids (Esin et al., 2021, 2022), demonstrate this evolutionary pattern in rivers.
For a better understanding of the nature of numerously proliferated polymorphisms of the mouth structure in Labeobarbus we have to study the evolutionary history of the Torinae lineages in the future. Mouth polymorphisms in relation to lip size including state ‘hyperthophied lips’ is common within the closely-related polyploid cyprinid lineages from the Middle East (e.g., ArabibarbusBorkenhagen, 2014 and Carasobarbus Karaman, 1971). This polymorphism is probably ancestral to both African and Middle East Torinae being inherited from Southern and Southeastern tetraploid Torinae lineages (e.g., Tor Gray, 1834 and NeolissochilusRainboth, 1985 – Roberts & Khaironizam, 2008; Yang et. al, 2015, 2022). Thus, a proliferation of a repeated, and predicted thick-lipped phenotype within Labeobarbus lineage might be due to an ancient genetic polymorphism that re-evolved under particular ecological circumstances. Given this, the thick-lipped phenotype ofLabeobarbus is obviously “pre-adaptive” upon emergencede-novo in various populations but not always functional, i.e., not yet necessarily involved in trophic resource partitioning according to our results (Figures 3B and 4). Nevertheless, prevalence of invertebrates in a diet or elevated d15N values in the thick-lipped ecomorph were discovered in all cases (including non-significant differences). This finding may be interpreted as a result of the generally adaptive value of the hypertrophied lips. In those populations that could not demonstrate an adaptive value of the hypertrophied lips in terms of trophic resource partitioning, the thickened lips might serve as a latent “adaptation in reserve” or “silent adaptation” that can be recruited into invertebrate foraging at appropriate environmental conditions possible due to being maintained in the population in low frequency and due to simple genetic and developmental programs that might also be facilitated by genetic assimilation based on initially phenotypic plasticity (Meyer, 1987; Schneider et al., 2014; Gunter et al., 2017).