Ohno S
Proc Natl Acad Sci U S A. 1981 Dec;78(12):7657-61. doi: 10.1073/pnas.78.12.7657.
The characteristic three-domain structure has been conserved throughout mammalian evolution by serum albumin and its fetal counterpart, alpha-fetoprotein. Thus, one still detects 35.2% amino acid sequence homology between bovine serum albumin and murine alpha-fetoprotein. Yet, natural selection cannot be invoked as the major factor responsible for the observed conservation of these sequences, for the simple reason of their dispensability. Inherited analbuminemia is apparently a harmless trait in man and the rat. The conservation appears inherent in their repetitious origin. Each protein is made of triplicate copies of the ancestral domain. Furthermore, analysis of the published sequence data suggests that the original coding sequence for the ancestral domain arose as repeats of the 18-base-long primordial building block sequence TTC-ACA-GAG-GAG-CAG-CTG specifying Phe-Thr-Glu-Glu-Gln-Leu and its shorter subsidiary TTC-ATG-GAG-GAG specifying Phe-Met-Glu-Glu. Consequently, the homology between bovine serum albumin and alpha-fetoprotein is mostly confined to small segments still specified by recognizable descendants of these building block sequences. The point to be made here is that evolutionary conservation of coding sequences can be an inherent property; natural selection need not be invoked.
血清白蛋白及其胎儿对应物甲胎蛋白在整个哺乳动物进化过程中保留了特征性的三结构域结构。因此，人们仍然可以检测到牛血清白蛋白和小鼠甲胎蛋白之间35.2%的氨基酸序列同源性。然而，不能将自然选择作为导致这些序列保守性的主要因素，原因很简单，这些序列是可有可无的。遗传性无白蛋白血症在人类和大鼠中显然是一种无害的性状。这种保守性似乎源于它们重复的起源。每种蛋白质都由祖先结构域的三份拷贝组成。此外，对已发表序列数据的分析表明，祖先结构域的原始编码序列是由18个碱基长的原始构建块序列TTC - ACA - GAG - GAG - CAG - CTG（对应苯丙氨酸 - 苏氨酸 - 谷氨酸 - 谷氨酸 - 谷氨酰胺 - 亮氨酸）及其较短的子序列TTC - ATG - GAG - GAG（对应苯丙氨酸 - 甲硫氨酸 - 谷氨酸 - 谷氨酸）重复产生的。因此，牛血清白蛋白和甲胎蛋白之间的同源性主要局限于这些构建块序列仍可识别的后代所指定的小片段。这里要指出的是，编码序列的进化保守性可能是一种固有属性；无需援引自然选择。
