To provide some objective data for accepting or refusing molecular clock hypothesis, non-parameter test method was employed based on mitochondrial cytochrome b gene sequence of six species of subfamily bovinae. The complete cytochrome b gene was 1 140 bp in length for all six bovine species and there was a little difference in base composition between species. Transition was the dominant base substitution model and the ratio of transition to transversion was 5.4. The testing results of relative evolution rate based on nucleotide sequences and amino acid sequences showed that molecular clock hypothesis was accepted absolutely within bovine species. The evolution of only a few sequences refused molecular clock hypothesis and evolution of most sequencesaccepted it among the species. It was easier to refuse molecular clock hypothesis based on the testing result from nucleotide sequence than the result from amino acid sequence. There was no obvious correlation between accepting or refusing molecular clock hypothesis and genetic distance between tested species. Molecular clock existed in some species. There was no nucleotide sequence or amino acid sequence varying in an absolutely stable rate in long evolution, and molecular clock was not unicersal.%在测定牛亚科家畜6个物种线粒体细胞色素b(Cyt b)基因全序列的基础上,以非参数检验法检验分子钟假说,提出肯定或否定分子钟假说的部分客观资料.结果表明,6个牛种的Cyt b基因全序列长度都是1140bp,牛种间序列的碱基组成差异较小,碱基替代以转换为主,转换/颠换比为5.4.基于核苷酸序列和氨基酸相对速率检验结果表明,牛种内序列的进化全部接受分子钟假说;牛种问大多数序列的进化接受分子钟假说,少数序列的进化拒绝分子钟假说.与基于氨基酸序列的检验结果相比较,基于核苷酸序列的检验结果更易于拒绝分子钟假说.进而推论,接受或者拒绝分子钟假说与所检测物种之间亲缘关系的远近无明显的相关性,分子钟假说在一定的物种范围内是成立的;对于长期进化而言,既无核苷酸序列亦无氨基酸序列以绝对恒定的速率变化,分子钟不具有通用性.
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