“Although it is widely recognized that ecological speciation can occur without gene flow between diverging groups of individuals , the recognition of its importance has grown because of recent evidence for speciation with gene flow . If gene flow commonly occurs during divergence, some mechanism, such as divergent selection must also occur frequently to counteract the homogenizing effect of gene flow. The manuscripts in this special issue, and a plethora of other recent publications [45–50], have made great strides in advancing our understanding of ecological speciation. These allow us to identify several key factors that affect progress toward ecological speciation, such as habitat choice (preference and avoidance), phenotypic plasticity, role of pollinators/parasites, complex biological interactions such as facilitation, as well as geographical context. However, for most cases, our understanding is still incomplete. For instance, the circumstances under which plasticity favors or inhibits adaptation, mate choice, and consequently ecological speciation are still largely unknown. Further insights will certainly arise from a multitude of empirical and theoretical studies, but certain areas of research are particularly likely to yield important results. For example, whereas we can rarely observe the time course of speciation in a single species, we can learn about factors affecting progress toward ecological speciation by studying and contrasting pairs of related populations at different points along the speciation continuum. Similarly, the study of parallel speciation may be highly informative. Such studies exist (e.g., [25, 26, 51, 52]), but we need many more systems where we can examine variation in progress toward ecological speciation. It is important that we also investigate instances where speciation fails, as these cases will advance our understanding of factors that constrain and enhance progress toward speciation. Furthermore, recent advances in DNA sequencing and statistical analysis offer an unprecedented opportunity to study the genetic basis and evolution of reproductive isolation during ecological speciation. The application of these new methods and models to ecologically well-studied systems have been and will be particularly informative [53, 54]. Finally, more studies using experimental manipulations to study the effects of key parameters on ecological speciation are badly needed, especially if they can be combined with an understanding of natural populations.”
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