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The idea that populations must be geographically isolated (allopatric) to evolve into separate species has persisted for a long time. It is now clear that new species can also diverge despite ongoing genetic exchange, but few accepted cases of speciation in sympatry have held up when scrutinized using modern approaches. Here, we examined evidence for speciation of the Howea palms of Lord Howe Island, Australia, in light of new genomic data. We used coalescence-based demographic models combined with double digest restriction site associated DNA sequencing of multiple individuals and provide support for previous claims by Savolainen et al. that speciation in Howea did occur in the face of gene flow., (© 2019 The Author(s). Evolution © 2019 The Society for the Study of Evolution.)

Although it is now widely accepted that speciation can occur in the face of continuous gene flow, with little or no spatial separation, the mechanisms and genomic architectures that permit such divergence are still debated. Here, we examined speciation in the face of gene flow in the Howea palms of Lord Howe Island, Australia. We built a genetic map using a novel method applicable to long-lived tree species, combining it with double digest restriction site-associated DNA sequencing of multiple individuals. Based upon various metrics, we detected 46 highly differentiated regions throughout the genome, four of which contained genes with functions that are particularly relevant to the speciation scenario for Howea, specifically salt and drought tolerance., (© 2019 The Author(s). Evolution © 2019 The Society for the Study of Evolution.)

On Lord Howe Island, speciation is thought to have taken place in situ in a diverse array of distantly related plant taxa (Metrosideros, Howea and Coprosma; Proc. Natl Acad. Sci. USA 108, 2011, 13188). We now investigate whether the speciation processes were driven by divergent natural selection in each genus by examining the extent of ecological and genetic divergence. We present new and extensive, ecological and genetic data for all three genera. Consistent with ecologically driven speciation, outlier loci were detected using genome scan methods. This mechanism is supported by individual-based analyses of genotype-environment correlations within species, demonstrating that local adaptation is currently widespread on the island. Genetic analyses show that prezygotic isolating barriers within species are currently insufficiently strong to allow further population differentiation. Interspecific hybridization was found in both Howea and Coprosma, and species distribution modelling indicates that competitive exclusion may result in selection against admixed individuals. Colonization of new niches, partly fuelled by the rapid generation of new adaptive genotypes via hybridization, appears to have resulted in the adaptive radiation in Coprosma - supporting the 'Syngameon hypothesis'., (© 2013 The Authors. Journal of Evolutionary Biology © 2013 European Society For Evolutionary Biology.)