Community and Conservation Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, Box 11103, 9700 CC Groningen, The Netherlands.
Syst Biol. 2012 Mar;61(2):204-13. doi: 10.1093/sysbio/syr091. Epub 2011 Aug 26.
Phylogenetic trees show a remarkable slowdown in the increase of number of lineages towards the present, a phenomenon which cannot be explained by the standard birth-death model of diversification with constant speciation and extinction rates. The birth-death model instead predicts a constant or accelerating increase in the number of lineages, which has been called the pull of the present. The observed slowdown has been attributed to nonconstancy of the speciation and extinction rates due to some form of diversity dependence (i.e., species-level density dependence), but the mechanisms underlying this are still unclear. Here, we propose an alternative explanation based on the simple concept that speciation takes time to complete. We show that this idea of "protracted" speciation can be incorporated in the standard birth-death model of diversification. The protracted birth-death model predicts a realistic slowdown in the rate of increase of number of lineages in the phylogeny and provides a compelling fit to four bird phylogenies with realistic parameter values. Thus, the effect of recognizing the generally accepted fact that speciation is not an instantaneous event is significant; even if it cannot account for all the observed patterns, it certainly contributes substantially and should therefore be incorporated into future studies.
系统发育树显示，谱系数量的增加在接近现代时明显放缓，这种现象无法用具有恒定物种形成和灭绝率的标准物种形成-灭绝模型来解释。相反，物种形成-灭绝模型预测谱系数量会保持不变或加速增加，这种现象被称为“现代的拉力”。观察到的放缓归因于物种形成和灭绝率的非恒定性，这是由于某种形式的多样性依赖性（即物种水平的密度依赖性）造成的，但这种现象背后的机制仍不清楚。在这里，我们提出了一种替代解释，基于这样一个简单的概念，即物种形成需要时间才能完成。我们表明，这种“拖延”物种形成的想法可以纳入多样化的标准物种形成-灭绝模型中。拖延的物种形成-灭绝模型预测了系统发育树上谱系数量增长率的现实放缓，并为具有现实参数值的四个鸟类系统发育提供了引人注目的拟合。因此，认识到物种形成不是一个瞬间事件的普遍接受的事实的影响是显著的；即使它不能解释所有观察到的模式，但它肯定有很大的贡献，因此应该纳入未来的研究。
