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The transition from “well-marked varieties” into “well-defined species”
has puzzled evolutionary biologists ever since Darwin — especially when
extensive gene flow between incipient species is possible due to the lack
of physical barriers (sympatric speciation). Gene flow counteracts the
build-up of genome-wide differentiation, which is both a hallmark of
speciation and forms the underlying basis of irreversible reproductive
barriers (incompatibilities) that ultimately complete the speciation
process. Theory predicts that the genetic architecture of divergently
selected traits can influence whether sympatric speciation occurs.
However, empirical data to test this prediction remain rare and are often
difficult to synthesize across animal taxa due to idiosyncrasies in their
biology and evolutionary histories. Here, within a young species complex
of Neotropical cichlid fish (Amphilophus spp.), we analyzed genomic
divergence among populations and species, and the genetic architecture of
traits that have been suggested to be important for this divergence, by
generating a new genome assembly and re-sequencing 453 genomes. We found
that species differing in mono/oligogenic traits affecting ecological
performance and/or mate choice show remarkably localized genomic
differentiation. In contrast, differentiation between species that
diverged in polygenic traits is widespread and much higher overall,
consistent with the evolution of effective and stable genome-wide barriers
to gene flow. Thus, we conclude that simple trait architectures are not
always as conducive to speciation-with-gene-flow as previously suggested,
whereas, unexpectedly, polygenic architectures can promote rapid and
stable speciation in sympatry.
359 views reported since publication in 2020.