50 



The Nature of Biological Diversity 



A mutation of this kind is likely to result in the restriction of the 

 mutant to certain very circumscribed habitats, habitats in which the 

 necessary metabolite can be found ready made. Further stepwise 

 mutations may result in the loss of ability to produce yet other metab- 

 olites and the habitats available become even more restricted. This 

 can lead in the end to the grossest kind of parasitism, especially 

 among bacteria, and it is very possible that symbiosis and frank para- 

 sitism may be the end results of similar, serial loss mutations in 

 animals as well. 



Among bacteria at any rate, loss of synthetic ability is often a 

 stepwise process and can lead to the most exacting of nutritional 

 requirements; many amino acids and a larger or smaller number of 

 accessory growth factors are often required, and these can sometimes 

 only be found in the tissues or tissue exudates of a specific living host. 

 Such is the case in Streptococcus haemolyticus for example, one of 

 the most exacting organisms known. A case that has been studied in 

 much detail is that of coenzyme A synthesis by bacteria. Requirements 

 for the total synthesis of this substance (see Table 2 and Fig. 1 ) have 

 been established ( Snell, 1956). 



Table 2. Biosynthesis of coenzyme A by bacteria 



Organism 



Requirement 



Corynebacterium diphtheriae 

 Lactobacillus bidgaricus 

 Lactobacillus casei 

 Acetobacter suboxydans 

 Treponema pallidum 



^-alanine 

 pantetheine 

 pantothenic acid 

 pantoic acid 

 pantetheine-4'-phosphate 



source: Snell (1956). 



Yet advantageous mutations, which are probably much less fre- 

 quent, have clearly taken place over and over again; indeed they have 

 formed the ultimate basis of all adaptation and consequent evolution- 

 ary progress. It would seem, however, that among mutations leading 

 to gain, as judged by advantages of survival value, some seem to be 

 more probable than others. 



One outstanding example of a gain mutation that seems to be of a 

 rather probable character is the invention of hemoglobin, by far the 

 commonest of the four known respiratory pigments ( Lemberg and 

 Legge, 1949) . We can say '"probable" because, unlike chlorocruorin. 

 hemerythrin, and hemocyanin, hemoglobin has turned up over and 

 over again without the slightest reverence for taxonomy; it crops up 



