Data from: Neutral processes forming large clones during colonization of new areas

In species reproducing both sexually and asexually clones are often more common in recently established populations. Earlier studies have suggested that this pattern arises due to natural selection favouring generally or locally successful genotypes in new environments. Alternatively, as we show here, this pattern may result from neutral processes during species’ range expansions. We model a dioecious species expanding into a new area in which all individuals are capable of both sexual and asexual reproduction, and all individuals have equal survival rates and dispersal distances. Even under conditions that favour sexual recruitment in the long run, colonisation starts with an asexual wave. After colonisation is completed, a sexual wave erodes clonal dominance. If individuals reproduce more than one season, and with only local dispersal, a few large clones typically dominate for thousands of reproductive seasons. Adding occasional long-distance dispersal, more dominant clones emerge, but they persist for a shorter period of time. The general mechanism involved is simple: edge effects at the expansion front favour asexual (uniparental) recruitment where potential mates are rare. Specifically, our model shows that neutral processes (with respect to genotype fitness) during the population expansion, such as random dispersal and demographic stochasticity, produce genotype patterns that differ from the patterns arising in a selection model. The comparison with empirical data from a postglacially established seaweed species (Fucus radicans) shows that in this case a neutral mechanism is strongly supported.在兼具有性繁殖（sexual reproduction）与无性繁殖（asexual reproduction）的物种中，克隆（clones）种群在新近建立的群体中往往更为常见。此前的研究认为，该现象源于自然选择（natural selection）青睐新环境中普遍或本地适应性较强的基因型；而本研究表明，该现象也可能源于物种范围扩张过程中的中性过程（neutral processes）。我们构建了一个雌雄异株（dioecious）物种的模型，该物种正扩张至新区域，所有个体均可进行有性繁殖与无性繁殖，且所有个体的存活率与扩散距离（dispersal distances）均一致。即便长期来看环境更有利于有性繁殖补充（sexual recruitment），种群定殖（colonisation）过程仍以无性繁殖波（asexual wave）为起始；定殖完成后，有性繁殖波会逐渐削弱克隆优势（clonal dominance）。若个体可进行多季繁殖且仅存在局部扩散，则少数大型克隆通常会主导种群达数千个繁殖季（reproductive seasons）之久；若引入偶发的长距离扩散，则会出现更多优势克隆，但它们的存续时长会更短。其核心机制十分简单：扩张前沿（expansion front）的边缘效应（edge effects）会青睐潜在交配对象稀缺区域的无性（单亲）繁殖补充。具体而言，本模型表明，种群扩张过程中与基因型适合度（genotype fitness）无关的中性过程，如随机扩散与种群统计随机性（demographic stochasticity），所产生的基因型分布模式，与选择模型（selection model）所产生的模式存在差异。通过与冰期后定殖的海藻物种脆囊藻（Fucus radicans）的实证数据对比可知，该案例强烈支持中性进化机制的作用。