Phenotypic Variation in Infants, Not Adults, Reflects Genotypic Variation among Chimpanzees and Bonobos

Studies comparing phenotypic variation with neutral genetic variation in modern humans have shown that genetic drift is a main factor of evolutionary diversification among populations. The genetic population history of our closest living relatives, the chimpanzees and bonobos, is now equally well documented, but phenotypic variation among these taxa remains relatively unexplored, and phenotype-genotype correlations are not yet documented. Also, while the adult phenotype is typically used as a reference, it remains to be investigated how phenotype-genotye correlations change during development. Here we address these questions by analyzing phenotypic evolutionary and developmental diversification in the species and subspecies of the genus Pan. Our analyses focus on the morphology of the femoral diaphysis, which represents a functionally constrained element of the locomotor system. Results show that during infancy phenotypic distances between taxa are largely congruent with non-coding (neutral) genotypic distances. Later during ontogeny, however, phenotypic distances deviate from genotypic distances, mainly as an effect of heterochronic shifts between taxon-specific developmental programs. Early phenotypic differences between Pan taxa are thus likely brought about by genetic drift while late differences reflect taxon-specific adaptations.

已有针对现代人类表型变异（phenotypic variation）与中性遗传变异（neutral genetic variation）的比较研究表明，遗传漂变（genetic drift）是人群间进化分化（evolutionary diversification）的主要驱动因素。目前，我们现存最近的近亲——黑猩猩（chimpanzees）与倭黑猩猩（bonobos）的种群遗传历史已得到充分解析，但这些分类单元（taxa）间的表型变异仍鲜有研究涉及，且表型-基因型关联（phenotype-genotype correlations）尚未被系统报道。此外，尽管通常以成体表型（adult phenotype）作为参照，但表型-基因型关联在发育过程中会发生何种变化，仍有待探究。本研究通过分析人猿属（Pan）各物种及亚种的表型进化与发育分化，对上述问题展开探讨。本研究的分析重点为股骨骨干（femoral diaphysis）的形态特征——其属于运动系统（locomotor system）中受功能约束的结构。研究结果显示，在幼年期，类群间的表型距离（phenotypic distances）与非编码（中性）基因型距离（non-coding (neutral) genotypic distances）整体上高度一致。然而在个体发育（ontogeny）的后期阶段，表型距离逐渐偏离基因型距离，这主要是由不同类群特异性发育程序（taxon-specific developmental programs）之间的异时性偏移（heterochronic shifts）所导致的。因此，人猿属类群间的早期表型差异大概率由遗传漂变引起，而后期差异则反映了类群特异性的适应性演化（adaptations）。