The shape of a phylogenetic tree is defined by the sequence of speciation events, represented by its branching points, and extinctions, represented by branch interruptions. In a neutral scenario of parapatry and isolation by distance, species tend to branch off the original population one after the other, leading to highly unbalanced trees. In this case the degree of imbalance, measured by the normalized Sackin index, grows linearly with species richness. Here we claim that moderate values of imbalance for trees with large number of species can occur if the geographic distribution involves more than one deme (allopatry) and speciation is parapatric within demes. The combined values of balance (normalized Sackin index) and species richness provide an estimate of how many demes were involved in the process if it happened in such neutral scenario. We also show that the spatial division in demes moderately slows down the diversification process, portraying a neutral mechanism for structuring the branch length distribution of phylogenetic trees.

中文翻译：

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同种异体增加中性物种在辐射过程中系统发育树的平衡

系统发育树的形状由物种形成事件的顺序（由其分支点表示）和物种灭绝（由分支中断表示）定义。在中性的隔离和远距离隔离的情况下，物种往往会一个接一个地从原始种群中分叉出来，从而导致树木高度失衡。在这种情况下，通过归一化的萨金指数衡量的不平衡程度随物种丰富度线性增长。在这里，我们声称，如果地理分布涉及一个以上的毁灭性事件（异象混生），并且物种形成在毁灭性事件之内是准生的，那么具有大量物种的树木就会出现适度的失衡。平衡（Sackin指数标准化）和物种丰富度的组合值提供了一个估计，如果在这种中性场景中发生了该事件，那么该过程涉及多少个犯罪。