Routes to speciation in Littorina

地球上生物的多样性是通过谱系的进化分裂形成新物种（物种形成）而产生的。分离过程通常是漫长的，在分离的不同阶段涉及多种机制。它可能开始于自然选择在不同的种群中以相反的方向运作，导致局部适应。然而，当种群在空间上分离时，它也可能从种群之间的不兼容性开始。在任何一种情况下，必须进化出其他特征来减少成功的杂交，从而完成生殖隔离。这些不同的物种形成途径的相对重要性尚不清楚，但它们有望在不同种群的基因组中留下不同的分化模式，并可能与不同的历史有关。我们提议在一组海螺中测试对主要物种形成模式的预测。这个群体中的一个物种有多种形式，生活在非常不同的栖息地。然而，尽管在大小、形状和行为上存在许多差异，这些形式仍然能够成功地杂交，这可能是因为杂种在中间栖息地的适应性并不比它们的亲本低得多。矛盾的是，也有一些物种表现出很少或没有生态分离，最近才出现分化，但却表现出接近完全的生殖障碍。这使得非常直接地比较种群历史和非常密切相关的种群的基因组分化模式，这些种群显然通过不同的途径向物种形成发展。我们将测量海螺在海岸上的时空分离、繁殖时间的分离、配偶选择和杂交适应度的影响，以便我们了解当前生态型之间和物种之间隔离的本质。然后，我们将利用基因组重测序来评估全基因组的分化模式。这些基因数据可以用来测试种群历史的替代方案，从而询问是否在物种起源周围可能存在一段空间分离时期，但在生态型起源周围不太可能存在。这些数据还可以识别出基因组中具有强烈基因交换障碍的部分，以及具有过去选择特征的部分。这些将与参与卵生和产卵物种不同生殖系统发育和功能的基因有关，这是导致杂交品种不相容的一个候选特征。总之，这些数据将使我们能够确定分歧选择和不相容进化的结果，提供一个例子，将告知物种形成的一般理论以及进化生物学中的其他问题，特别是局部适应的遗传基础。

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.