Abstract
It is now recognized that speciation can proceed even when divergent natural selection is opposed by gene flow. Understanding the extent to which environmental gradients and geographical distance can limit gene flow within species can shed light on the relative roles of selection and dispersal limitation during the early stages of population divergence and speciation. On the remote Lord Howe Island (Australia), ecological speciation with gene flow is thought to have taken place in several plant genera. The aim of this study was to establish the contributions of isolation by environment (IBE) and isolation by community (IBC) to the genetic structure of 19 plant species, from a number of distantly related families, which have been subjected to similar environmental pressures over comparable time scales. We applied an individual-based, multivariate, model averaging approach to quantify IBE and IBC, while controlling for isolation by distance (IBD). Our analyses demonstrated that all species experienced some degree of ecologically driven isolation, whereas only 12 of 19 species were subjected to IBD. The prevalence of IBE within these plant species indicates that divergent selection in plants frequently produces local adaptation and supports hypotheses that ecological divergence can drive speciation in sympatry.
Highlights
The aim of this study was to establish the contributions of isolation by environment (IBE) and isolation by community (IBC) to the genetic structure of 19 plant species, from a number of distantly related families, which have been subjected to similar environmental pressures over comparable time scales
The role of natural selection in speciation has received renewed interest owing to the growing body of data that has demonstrated the potential for divergent selection to overcome the homogenizing effects of gene flow between populations [1,2,3,4,5,6]
Selection against migrants and hybrids can yield a pattern similar to that stemming from isolation by distance (IBD), where increasing environmental dissimilarity between locations is correlated with neutral genetic divergence between populations or individuals [14]
Summary
The role of natural selection in speciation has received renewed interest owing to the growing body of data that has demonstrated the potential for divergent selection to overcome the homogenizing effects of gene flow between populations [1,2,3,4,5,6]. Selection against migrants and hybrids can yield a pattern similar to that stemming from IBD, where increasing environmental dissimilarity between locations is correlated with neutral genetic divergence between populations or individuals [14] This pattern of isolation by environment (IBE) is increasingly seen as a signature or precursor to incipient ecological speciation [13,15]. Previous research [8,26,27] suggests that nine of the endemic species (from the genera Coprosma, Metrosideros and Howea) are the products of speciation with gene flow that has occurred on LHI (referred to as the ‘sympatric speciation group’, SSG) These within-island speciation events can be considered as sympatric under the biogeographic definition that we use here; under strict population genetic definitions, these events may be considered as parapatric speciation [26,30,31]. Each species was analysed separately, with the exception of the two Macropiper species which were analysed together to detect interspecific hybridization between these close relatives
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