Adaptive radiation of fishes was long thought to be possible only in
lacustrine environments. Recently, several studies have shown that also
riverine and stream environments provide the ecological opportunity for
adaptive radiation. In this study, we report on a riverine adaptive
radiation of six ecomorphs of cyprinid hillstream fishes of the genus
Garra in a river located in the Ethiopian Highlands in East
Africa. Garra are predominantly highly specialized algae-scrapers
with a wide distribution ranging from Southeastern Asia to Western
Africa. However, adaptive phenotypic diversification in mouth type,
sucking disc morphology, gut length and body shape have been found among
these new species in a single Ethiopian river. Moreover, we found two
novel phenotypes of Garra (‘thick-lipped’ and ‘predatory’) that
were not described before in this species-rich genus (>160
species). Mitochondrial and genome-wide data suggest monophyletic,
intra-basin evolution of Garra phenotypic diversity with
signatures of gene flow from other local populations. Although sympatric
ecomorphs are genetically distinct and can be considered to being young
species as suggested by genome-wide SNP data, mtDNA was unable to
identify any genetic structure suggesting a recent and rapid speciation
event. Furthermore, we found evidence for a hybrid origin of the novel
‘thick-lipped’ phenotype, as being the result of the hybridization of
two other sympatrically occurring species. Here we highlight how, driven
by ecological opportunity, an ancestral trophically highly specialized
lineage is likely to have rapidly adaptively radiated in a riverine
environment, and that this radiation was promoted by the evolution of
novel feeding strategies.
Adaptive Radiation, Ecological Opportunity, and Evolutionary Determinism