Departamento de Ciencias Biológicas, Universidad de los Andes, Bogotá, Colombia.
Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA, USA.
Syst Biol. 2018 Mar 1;67(2):181-194. doi: 10.1093/sysbio/syx071.
Progress in the development and use of methods for species delimitation employing phenotypic data lags behind conceptual and practical advances in molecular genetic approaches. The basic evolutionary model underlying the use of phenotypic data to delimit species assumes random mating and quantitative polygenic traits, so that phenotypic distributions within a species should be approximately normal for individuals of the same sex and age. Accordingly, two or more distinct normal distributions of phenotypic traits suggest the existence of multiple species. In light of this model, we show that analytical approaches employed in taxonomic studies using phenotypic data are often compromised by three issues: 1) reliance on graphical analyses that convey little information on phenotype frequencies; 2) exclusion of characters potentially important for species delimitation following reduction of data dimensionality; and 3) use of measures of central tendency to evaluate phenotypic distinctiveness. We outline approaches to overcome these issues based on statistical developments related to normal mixture models (NMMs) and illustrate them empirically with a reanalysis of morphological data recently used to claim that there are no morphologically distinct species of Darwin's ground-finches (Geospiza). We found negligible support for this claim relative to taxonomic hypotheses recognizing multiple species. Although species limits among ground-finches merit further assessments using additional sources of information, our results bear implications for other areas of inquiry including speciation research: because ground-finches have likely speciated and are not trapped in a process of "Sisyphean" evolution as recently argued, they remain useful models to understand the evolutionary forces involved in speciation. Our work underscores the importance of statistical approaches grounded on appropriate evolutionary models for species delimitation. We discuss how NMMs offer new perspectives in the kind of inferences available to systematists, with significant repercussions on ideas about the phenotypic structure of biodiversity.
基于表型数据进行物种界定的方法的发展和应用的进展落后于分子遗传方法的概念和实际进展。在使用表型数据来界定物种时，基本的进化模型假设随机交配和数量多基因特征，因此同一性别和年龄的个体的表型分布应该近似正态分布。因此，两个或更多个不同的表型特征正态分布表明存在多个物种。根据这个模型，我们表明，在使用表型数据进行分类学研究中使用的分析方法常常受到三个问题的影响：1）依赖于图形分析，这些分析很少传达有关表型频率的信息；2）在降低数据维度后，排除可能对物种界定很重要的特征；3）使用集中趋势的度量来评估表型的独特性。我们概述了基于与正态混合模型（NMM）相关的统计发展来克服这些问题的方法，并通过对最近用于声称达尔文地雀（Geospiza）没有形态上明显物种的形态数据的重新分析来举例说明。我们发现，相对于承认多种物种的分类学假设，这种说法几乎没有得到支持。尽管需要使用其他信息来源对地雀物种界限进行进一步评估，但我们的结果对包括物种形成研究在内的其他研究领域具有影响：因为地雀可能已经形成了物种，而不像最近有人认为的那样被困在“西西弗斯式”进化的过程中，因此它们仍然是理解物种形成中涉及的进化力量的有用模型。我们的工作强调了基于适当进化模型的统计方法对于物种界定的重要性。我们讨论了 NMM 如何为系统学家提供新的观点，这对关于生物多样性的表型结构的想法产生了重大影响。
