Strauss André, Hubbe Mark
Departamento de Genética e Biologia Evolutiva, Institute de Biociências, Universidade de São Paulo, Rua do Matão 277, 05508-090 São Paulo, Brazil.
Hum Biol. 2010 Jun;82(3):315-30. doi: 10.3378/027.082.0305.
The statement that pairs of individuals from different populations are often more genetically similar than pairs from the same population is a widespread idea inside and outside the scientific community. Witherspoon et al. ["Genetic similarities within and between human populations," Genetics 176:351-359 (2007)] proposed an index called the dissimilarity fraction (omega) to access in a quantitative way the validity of this statement for genetic systems. Witherspoon demonstrated that, as the number of loci increases, omega decreases to a point where, when enough sampling is available, the statement is false. In this study, we applied the dissimilarity fraction to Howells's craniometric database to establish whether or not similar results are obtained for cranial morphological traits. Although in genetic studies thousands of loci are available, Howells's database provides no more than 55 metric traits, making the contribution of each variable important. To cope with this limitation, we developed a routine that takes this effect into consideration when calculating omega. Contrary to what was observed for the genetic data, our results show that cranial morphology asymptotically approaches a mean omega of 0.3 and therefore supports the initial statement--that is, that individuals from the same geographic region do not form clear and discrete clusters--further questioning the idea of the existence of discrete biological clusters in the human species. Finally, by assuming that cranial morphology is under an additive polygenetic model, we can say that the population history signal of human craniometric traits presents the same resolution as a neutral genetic system dependent on no more than 20 loci.
来自不同群体的个体对往往比来自同一群体的个体对在基因上更相似，这一说法在科学界内外都是一个广泛存在的观点。威瑟斯庞等人[《人类群体内部和之间的基因相似性》，《遗传学》176:351 - 359(2007)]提出了一种称为差异分数(ω)的指数，以定量评估这一说法对于基因系统的有效性。威瑟斯庞证明，随着基因座数量的增加，ω会降低到一个点，在有足够样本的情况下，该说法是错误的。在本研究中，我们将差异分数应用于豪威尔斯的颅骨测量数据库，以确定对于颅骨形态特征是否能得到类似的结果。虽然在基因研究中有数千个基因座可用，但豪威尔斯的数据库提供的测量特征不超过55个，这使得每个变量的贡献都很重要。为了应对这一限制，我们开发了一种程序，在计算ω时会考虑到这种影响。与基因数据的情况相反，我们的结果表明，颅骨形态渐近地接近平均ω为0.3，因此支持了最初的说法——即来自同一地理区域的个体不会形成清晰且离散的聚类——这进一步质疑了人类物种中离散生物聚类存在的观点。最后，通过假设颅骨形态处于加性多基因模型之下，我们可以说人类颅骨测量特征的群体历史信号呈现出与依赖不超过20个基因座的中性基因系统相同的分辨率。
