Mitochondrial cytochrome b (cytb) is   among the most extensively sequenced genes to date across the vertebrates. Here,   we employ nearly 2,000 cytb gene sequences from GenBank to calculate and   compare levels of genetic distance between sister species, congeneric   species, and confamilial genera within and across the major vertebrate taxonomic   classes. The results of these analyses parallel and reinforce some of the   principal trends in genetic distance estimates previously reported in a   summary of the multilocus allozyme literature. In particular, surveyed avian   taxa on average show significantly less genetic divergence than do same-rank   taxa surveyed in other vertebrate groups, notably amphibians and reptiles.   Various biological possibilities and taxonomic ‘‘artifacts’’ are considered   that might account for this pattern. Regardless of the explanation, by the   yardstick of genetic divergence in this mtDNA gene, as well as genetic   distances in allozymes, there is rather poor equivalency of taxonomic rank   across some of the vertebrates.

线粒体细胞色素B（cytb）是迄今为止脊椎动物中测序最广泛的基因之一。在这里，我们利用Genbank的近2000个cytb基因序列计算并比较了主要脊椎动物类群中姊妹种、同属种和同科不同属之间的遗传距离水平。这些分析的结果与先前基于多基因座同工酶研究报道的遗传距离差异趋势相似并加强了这种趋势。特别是，被调查的鸟类之间的遗传距离明显小于其他脊椎动物群体内种间的遗传距离，特别是两栖动物和爬行动物。这种模式可能解释了以前的多种生物可能性和分类上“人为偏差”。不管怎么解释，根据线粒体基因的遗传分化程度以及同工酶的遗传距离，脊椎动物不同门类之间的分类等级的等效性较差。