Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

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POSTSCRIPT 4^1 a substance A is in itself unlikely to be of value in deciding how organisms are related. The likelihood is perhaps increased if the substance had no function in the organisms in question, when it could be considered as analogous to a vestigial organ. When taken in the context of the com¬ parative morphology of the animals in which they are found, substances such as arginine and creatine phosphates have an interest. But this interest is not primarily that their occurrence demonstrates a relationship, but that the relationships demonstrated by comparative morphology afford some degree of explanation of the sporadic occurrence of the compounds. We are justified in thinking that at certain stages of evolution a compound is useful to a group of animals, but we are not justified in thinking that the process of synthesizing the compound was not available (i.e. had not evolved) until that group of animals evolved. Is it possible that high molecular weight compounds, proteins, poly¬ saccharides and nucleic acids, will be more informative than are the low molecular weight compounds about the ancestry of animals? Serological studies have in general supported many of the relationships postulated by comparative morphologists. But it is not certain that the serological evidence is really independent evidence. For example, Wilhelmi (1942) found that the antigens of hemichordates resemble echinoderm antigens much more than they do mollusc or arthropod antigens. But this would only constitute independent evidence of relatedness if we were certain that particular types of morphologies do not obligatorily imply particular types of antigens. If we could prove that it is possible to have a hemichordate with antigens more resembling mollusc antigens than echinoderm antigens, then the serological evidence is independent evidence of relatedness. But if hemichordate structure can only be based upon certain types of antigens, then serological relations are merely another aspect of morphological relations. Apart from the question as to the significance of serological evidence, there is some evidence suggesting that the ability to synthesize large molecules of specific function also is ubiquitous. For example, it seems quite clear that haemoglobin has appeared independently in a number of only very distantly related organisms. It is therefore relevant to ask whether the different types of protein, nucleic acid and polysaccharide are not also, like the low molecular weight compounds, part of the common chemical repertoire of most animals, and consequently available at need in the course of evolution. Medawar has written (p. 334): '...endocrine evolution is not an evolution of hormones but an evolution of the uses to which they are put; an evolution not, to put it crudely, of chemical formulae but of reactivities, reaction patterns and tissue competences.' In