Parallel evolution can be expected among closely related taxa exposed to similar selective pressures. However, parallelism is typically stronger at the phenotypic level, while genetic solutions to achieve these phenotypic similarities may differ. For polygenic traits, the availability of standing genetic variation (i.e., heterozygosity) may influence such genetic nonparallelism. Here, we examine the extent to which high-elevation adaptation is parallel—and whether the level of parallelism is affected by heterozygosity—by analyzing genomes of 19 Paridae species distributed across East Asia with a dramatic east–west elevation gradient. We find that western highlands endemic parids have consistently lower levels of heterozygosity—likely the result of late-Pleistocene demographic contraction—than do parids found exclusively in eastern lowlands, which remained unglaciated during the late Pleistocene. Three widespread species (east to west) have high levels of heterozygosity similar to that observed in eastern species, although their western populations are less variable than eastern ones. Comparing genomic responses to extreme environments of the Qinghai–Tibet Plateau, we find that the most differentiated genomic regions between each high-elevation taxon and its low-elevation relative are significantly enriched for genes potentially related to the oxygen transport cascade and/or thermogenesis. Despite no parallelism at particular genes, high similarity in gene function is found among comparisons. Furthermore, parallelism is not higher in more heterozygous widespread parids than in highland endemics. Thus, in East Asian parids, parallel functional response to extreme elevation appears to rely on different genes, with differences in heterozygosity having no effect on the degree of genetic parallelism.

在暴露于相似选择压力的密切相关的分类群中，可以预期平行进化。然而，平行性通常在表型水平上更强，而实现这些表型相似性的遗传解决方案可能会有所不同。对于多基因性状，现有遗传变异（即杂合性）的可用性可能会影响这种遗传非平行性。在这里，我们通过分析分布在东亚的 19 个 Paridae 物种的基因组，以显着的东西向海拔梯度来检验高海拔适应的平行程度以及平行程度是否受到杂合性的影响。我们发现西部高地特有的parids的杂合性水平始终较低 - 可能是晚更新世人口收缩的结果 - 比仅在东部低地发现的parids，在晚更新世期间仍然没有冰川。三个广泛分布的物种（从东到西）具有与在东部物种中观察到的相似的高水平杂合性，尽管它们的西部种群比东部种群的变化小。比较对青藏高原极端环境的基因组反应，我们发现每个高海拔分类群与其低海拔相关物种之间差异最大的基因组区域显着富集了可能与氧气运输级联和/或产热相关的基因。尽管在特定基因上没有平行性，但在比较中发现了基因功能的高度相似性。此外，杂合广泛分布的parids的平行度并不高于高地地方病。因此，在东亚parids中，