Some lineages radiate spectacularly when colonising a region, but others do not. Large radiations are often attributed to species’ adaptation into niches, but sometimes instead to other drivers, such as biogeography. Here we aim to disentangle the factors determining radiation size, by modelling simplified scenarios without the complexity of explicit niches. We build a spatially structured neutral model free from niches and incorporating a form of protracted speciation that accounts for gene flow between populations. We characterise the behaviour of the model for a range of different networks of connectivity between patches. We find that a wide range of radiation sizes are possible depending on the combination of geographic isolation and species' dispersal ability. For example, when considering isolated archipelagos, low rates of dispersal from the mainland result in decreased competition and thus increased radiation size. Dispersal between habitat patches also has an important effect. At extremely low dispersal rates, each habitat patch has its own endemic species, intermediate dispersal rates foster larger radiations. As dispersal rates increase further, a critical point is reached at which identical lineages can vary greatly in radiation size due to rare and stochastic dispersal events. At the critical point, some lineages remain a single species for a long time, whilst others with identical characteristics produce the largest radiations of all. The mechanism for this is a ‘radiation cascade’ in which speciation leads to reduced numbers of individuals per species, and thus reduced gene flow between conspecifics in isolated patches, leading to yet more speciation. Once a radiation cascade begins, it continues rapidly until it is arrested by a new equilibrium between speciation and extinction. We speculate that such cascades may occur more generally and are not only present in neutral models. This may help to explain rapid radiation, and the extreme radiation sizes of certain lineages. Whilst niches no doubt play a role in community assembly, our findings lead us to question if diversification and adaptation into niches is sometimes an effect of speciation and rapid radiation, rather than its cause.

中文翻译：

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进化辐射的生物地理驱动因素

有些世系在殖民一个地区时会散发出惊人的光芒，但其他世系则不然。大辐射通常归因于物种对生态位的适应，但有时也归因于其他驱动因素，例如生物地理学。在这里，我们的目标是通过在没有显式壁龛复杂性的情况下对简化场景进行建模，来解开决定辐射大小的因素。我们构建了一个没有生态位的空间结构中性模型，并结合了一种解释种群之间基因流动的长期物种形成形式。我们描述了模型在补丁之间的一系列不同连接网络中的行为。我们发现，根据地理隔离和物种传播能力的组合，可能会有多种辐射大小。例如，在考虑孤立的群岛时，大陆的低扩散率导致竞争减少，从而增加辐射规模。栖息地斑块之间的分散也有重要影响。在极低的扩散率下，每个栖息地都有自己的特有物种，中等的扩散率会产生更大的辐射。随着扩散率的进一步增加，达到了一个临界点，在这个临界点上，由于罕见和随机的扩散事件，相同谱系的辐射大小可能会有很大差异。在临界点，一些谱系长期保持单一物种，而其他具有相同特征的谱系产生最大的辐射。其机制是“辐射级联”，其中物种形成导致每个物种的个体数量减少，从而减少了孤立斑块中同种物种之间的基因流动，导致更多的物种形成。一旦辐射级联开始，它会迅速持续，直到它被物种形成和灭绝之间的新平衡所阻止。我们推测这种级联可能会更普遍地发生，而不仅出现在中性模型中。这可能有助于解释快速辐射和某些谱系的极端辐射大小。虽然生态位无疑在群落组装中发挥作用，但我们的研究结果让我们质疑，多样化和适应生态位有时是物种形成和快速辐射的结果，而不是其原因。以及某些谱系的极端辐射大小。虽然生态位无疑在群落组装中发挥作用，但我们的研究结果让我们质疑，多样化和适应生态位有时是物种形成和快速辐射的结果，而不是其原因。以及某些谱系的极端辐射大小。虽然生态位无疑在群落组装中发挥作用，但我们的研究结果让我们质疑，多样化和适应生态位有时是物种形成和快速辐射的结果，而不是其原因。