Morphology versus Molecules in Evolution

One of the first investigators of the “blood relationship” of organisms was George Henry Falkiner Nuttall. Early in the 20th century, Nuttall sought to demonstrate that the degree of similarity between animals in their blood serum proteins reflected their evolutionary closeness. He produced an antiserum to serum of one animal and combined it with another animal's blood serum, producing a precipitate. The more precipitate that formed, the greater the similarity (due to a greater number of antibody–antigen binding sites) and, consequently, the closer the supposed evolutionary relationship between the organisms [Page 1627]being compared. Nuttall also thought that the reaction rate reflected closeness of relatedness. He concluded in Blood Immunity and Blood Relationship, “If we accept the degree of blood reaction as an index of the degree of blood relationship within the Anthropoidea, then we find that the Old World apes are more closely allied to man than are the New World apes, and this is exactly in accordance with the opinion expressed by Darwin.” But for the most part,Nuttall's work went unrecognized for decades.

Perhaps the most influential publication of the early 1960s in the area of biochemistry and relatedness was Emile Zuckerkandl and Linus Pauling's 1962 chapter on fetal and adult hemoglobin. They compared human, gorilla, horse, and fish sequences and found that the fish was more dissimilar to the human than to the horse and that the gorilla was most similar to the human. Because this arrangement mirrored the morphologically based phylogeny of these taxa, Zuckerkandl and Pauling believed that their observations “can be understood at once if it is assumed that in the course of time the hemoglobin-chain genes duplicate, [and] that the descendants of the duplicate genes ‘mutate away’ from each other” (emphasis added). From this assumption, they concluded that “over-all similarity must be an expression of evolutionary history,” with descendants “mutating away” and becoming “gradually more different from each other.” That is, the more ancient a lineage, the more molecular difference its descendants will gradually accumulate. Consequently, evolutionary closeness became equated with molecular difference despite the fact that Zuckerkandl and Pauling's study did not justify assuming that “over-all similarity” is “an expression of evolutionary history,” that difference always reflects the same directionality of change, or that molecular change accrues at a constant and gradual rate. Nevertheless, as Adalgisa Caccone and Jeffrey R. Powell wrote in 1989, “Virtually all molecular phylogenetic studies…have a major underlying assumption: The genetic similarity or difference among taxa is an indication of phylogenetic relatedness. Lineages that diverged more recently will be genetically more similar to one another than will be lineages with more ancient splits.” Embrace the assumption and the rest may follow logically, but not necessarily biologically.

In 1966, Vincent Sarich and Allan Wilson began investigating primate evolutionary relationships using albumin, translating degree of reactivity between albumin and anti-albumin into “immunological distance” (ID). An ID value of 1.0 meant “identical” (as in combining serum and antiserum of the same individual). The closer an ID value was to 1.0, the greater the overall molecular similarity and thus, by extension, the more closely related the taxa under study. As a “check” on the validity of this approach, Sarich and Wilson conceived the “test of reciprocity” (reversing whose serum or antiserum was used), which usually confirmed the initial finding. Because their arrangement of the primates agreed more with than it differed from the morphologically based pattern of primate relationships, they concluded that the “data are in qualitative agreement with the anatomical evidence, on the basis of which the apes, Old World monkeys, New World monkeys, prosimians, and nonprimates are placed in taxa which form a series of decreasing genetic relationship to man.”

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