Repeated and predictable patterns of ecotypic differentiation during a biological invasion: lake-stream divergence in parapatric Swiss stickleback

摘要

The relative importance of ecological selection and geographical isolation in\udpromoting and constraining genetic and phenotypic differentiation among\udpopulations is not always obvious. Interacting with divergent selection,\udrestricted opportunity for gene flow may in some cases be as much a cause\udas a consequence of adaptation, with the latter being a hallmark of ecologi-\udcal speciation. Ecological speciation is well studied in parts of the native\udrange of the three-spined stickleback. Here, we study this process in a\udrecently invaded part of its range. Switzerland was colonized within the past\ud140 years from at least three different colonization events involving differ-\udent stickleback lineages. They now occupy diverse habitats, ranging from\udsmall streams to the pelagic zone of large lakes. We use replicated systems\udof parapatric lake and stream populations, some of which trace their origins\udto different invasive lineages, to ask (i) whether phenotypic divergence\udoccurred among populations inhabiting distinct habitats, (ii) whether trajec-\udtories of phenotypic divergence follow predictable parallel patterns and (iii)\udwhether gene flow constrains divergent adaptation or vice versa. We find\udconsistent phenotypic divergence between populations occupying distinct\udhabitats. This involves parallel evolution in several traits with known eco-\udlogical relevance in independent evolutionary lineages. Adaptive divergence\udsupersedes homogenizing gene flow even at a small spatial scale. We find\udevidence that adaptive phenotypic divergence places constraints on gene\udflow over and above that imposed by geographical distance, signalling the\udearly onset of ecological speciation.