Routes to speciation in Littorina

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The diversity of organisms on earth has arisen through the evolutionary splitting of lineages to form new species (speciation). The splitting process is often lengthy, involving multiple mechanisms at different stages in separation. It may begin with natural selection operating in opposite directions in different populations, causing local adaptation. However, it may also begin with incompatibilities between populations that arise when populations are spatially separated. In either case, other traits must evolve that reduce successful interbreeding, thus completing reproductive isolation. The relative importance of these different pathways to speciation is poorly understood but they are expected to leave different patterns of differentiation in the genomes of diverging populations and may be associated with different histories. We propose to test predictions of the major modes of speciation in a group of marine snails. One species in this group has multiple forms living in very distinct habitats. However, despite many differences in size, shape and behaviour, these forms are still able to interbreed successfully, perhaps because hybrids do not have much lower fitness than their parents in intermediate habitats. Paradoxically, there are also species that show little or no ecological separation, have diverged recently and yet show close to complete reproductive barriers. This allows a very direct comparison of population histories and patterns of genomic differentiation in very closely related populations that have, apparently, progressed towards speciation by different routes. We will measure the effects of spatial and temporal separation of snails on the sea shore, separation in timing of reproduction, mate choice and hybrid fitness so that we know the nature of current isolation between ecotypes and between species. We will then make use of genome re-sequencing to assess the genome-wide pattern of differentiation. These genetic data can be used to test alternative scenarios for the history of the populations and so to ask whether a period of spatial separation was likely around the origin of the species but unlikely around the origin of the ecotypes. The data can also identify parts of the genome with strong barriers to gene exchange and with signatures of past selection. These will be linked to genes involved in the development and function of the distinct reproductive systems of the egg-laying and brooding species, a candidate trait for contributing to incompatibility in hybrids. Together, these data will allow us to determine the outcomes of divergent selection and evolution of incompatibility, providing an example that will inform general theories of speciation as well as other questions in evolutionary biology, particularly the genetic basis of local adaptation.