SESSION V. DISCUSSION 497 



us an idea of the time during which evolution has been going on on our planet and, hence, 

 also of the time of origin of life. 



As well as the considerations brought forward by Pauling as a basis for his calculations, 

 namely the significance of selection, the rate of replacement of generations and relative 

 numbers of normal and mutated allelomorphs, we must include the pressure of the 

 mutational process in the calculation. There is a type of mutation of which it may 

 reasonably be assvmied that there is a single appearance of each particular type. These 

 are inversions and for their appearance they require the concurrence of two genetic 

 events. Inversions are now widely used for the study of the courses of evolution of Diptera. 

 They can be used for calculation of the rate of evolution. 



So-called point mutations or genovariations arise repeatedly. The repeated occurrence 

 of a mutation cuts down the ages which must have passed according to Pauling's calcu- 

 lations. These ages are still further curtailed if we reckon that this mutability is, itself, 

 being attained by the action of selection. The work of a number of authors, in particular 

 Timofeev-Risovskiï, Demerec, Mempell, Tinyakov and many others, has shown that 

 there are genotypic causes which determine the frequency of the appearance of mutations. 

 But this means that selection in favour of a mutant impUes selection in favour of muta- 

 bility and, therefore, a distribution in the normal part of the population, which does not 

 contain the mutation, of the genotypic causes which bring about increased mutability in 

 the same direction as selection of the mutant. This view is supported by experimental 

 evidence. By artificial selection it is possible to imitate the processes which occur in 

 populations imder natural conditions. Two methods are available, imitation of individual 

 selection and imitation of the selection of groups. We have succeeded in bringing about 

 an increase in mutability in Drosophila by both methods. In particular, together with 

 Pomerantseva, we selected from natural populations mutant males, carrying sex-linked 

 mutations and compared the frequency of the appearance of lethal, semilethal and visible 

 mutations in their genotype and in that of normal males. The mutability of the mutants 

 was higher than that of the normal individuals. 



In another experiment the selection of mutable lines, i.e. the selection of groups, led 

 to an increase in mutability. Thus, mutability is achieved by artificial selection and this 

 lends probability to the suggestion that it is also achieved by natural selection and, if this 

 is so, it supports the hypothesis of the acceleration of the tempo of evolution. The accelera- 

 tion of the tempo of evolution must be taken into account in calculations of the length of 

 time during which evolution has been taking place. What I have been speaking of in 

 connection with the interdependence of mutabiUty and selection is only one of the factors 

 affecting the acceleration of evolution; however, this example demonstrates, not merely 

 the acceleration of evolution, but also the acceleration of this acceleration. 



V. S. ToNGUR (U.S.S.R.): 



In considering the problem of the origin of structure and metabolism, the question 

 immediately and inevitably arises as to whether metabolism appears along with the origin 

 of structure in the chemical sense, i.e. is the structure of high-polymers, even such com- 

 plex ones as proteins, nucleic acids and nucleoproteins, sufficient, or in some sort of 

 cytological structure necessary for the origin of metabolism ? It would seem that this ques- 

 tion cannot be answered at present. To judge from the papers and contributions to this 

 Symposiimi, both points of view have their supporters. The evidence required for the 

 solution of this problem, the general biological importance of which it is hard to reassess, 

 should be provided by experiments on the disappearance of metaboUsm with the de- 

 gradation of cytoplasmic structures. Together with A. M. Zubovskaya, I have carried 

 out such experiments at the Institute of Experimental Biology. 



The work was carried out on the granules isolated from homogenates of the livers of 

 rats by differential centrifugation at 5000 g. These granules are, in fact, a mixture of 

 large Light granules and the mitochondria proper. In what follows, they wiU be provi- 

 sionally referred to as mitochondria. The isolated granules were disrupted by lysis in 

 distilled water and we studied their ability to synthesize protein in the presence of a 



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