Section 9 — Population Genetics 



Such a tremendous change in the occurrence 

 rate of visible mutations was not observed in 

 other loci of the sex-chromosome. A certain 

 decrease in the rate of occurrence of sex-linked 

 lethals was also observed in some populations. 



Synchronous mutability changes in geogra- 

 phically isolated populations involving such an 

 extensive area ought probably be considered 

 as caused by some external geophysical or cos- 

 mical factor, influencing differently the mutabili- 

 ty of different loci. We can only guess at the 

 nature of the factor which may act either di- 

 rectly or through the medium of some bio- 

 ecological changes. 



9.41. On the Evolutionary Signification of Heterozy- 

 gosity in Drosophila Populations. E. Bosiger 

 (Gif-sur-Yvette, France). 



Natural populations of Drosophila melano- 

 gaster show a high degree of heterozygosity. 



The vigour of males of stocks recently derived 

 from natural populations is higher than that of 

 inbred laboiatory stocks. Inbreeding lowers the 

 vigour. Hybridization of two stocks of low 

 vigour produces very vigorous males. This 

 hybrid vigour is maintained for at least sixty 

 generations in populations of at least 200 flies. 



The advantage of heterozygous males in in- 

 traspecific sexual selection constitutes a mecha- 

 nism of genetic homeostasis which maintains the 

 high degree of heterozygosity of natural popu- 

 lations. Heterozygous flies produce more eggs 

 and spermatozoa and resist moulds and bacteria 

 better. 



A high degree of heterozygosity seems to be 

 important for natural populations of Drosophila, 

 which are very reduced in winter. The mainte- 

 nance of a rich gene pool by heterozygosity avoids 

 rapid changes of the genetical constitution by 

 genetic drift, and enables the species eventually 

 to undergo rapid expansion or an adaptation to 

 changing conditions. 



This work on intraspecific sexual selection 

 and on the comparison of the fecundity of homo- 

 zygous and heterozygous flies leads to a new 

 technique of augmentation of the vigour in 

 inbred lines without loss of the acquired selection 

 profit. 



The enrichment of the morphological, physio- 

 logical and psychological possibilities of a species 

 by the creation of new loci seems to be the major 

 progress of evolution and could be more im- 

 portant than the substitution of alleles at existing 

 loci by mutation. 



9.42. The Effect of Inbreeding on the "Fitness" and 

 on the Rate of Attainment of Acclimatization 

 to Temperature in Drosophila subobscura. 

 K. Bowler and M. J. Hollingsworth 

 (London, Great Britain). 



A further aspect of the hypothesis that in- 

 breeding leads to a loss of "biochemical ver- 

 satility"d) has been tested. Males from the 

 B & K inbred lines of Drosophila subobscura and 

 from the recriprocal hybrids between these in- 

 bred lines were used. ( -> The criterion of bio- 

 chemical versatility has been the rate at which 

 these males can adapt to changes in temperature. 

 Fitness has also been measured by comparing the 

 ability of the inbreds and hybrids to withstand a 

 high lethal temperature (34°C). All flies were 

 fully acclimatized at either 14.5°C or 25°C and 

 then placed at the reverse acclimatization tem- 

 perature for periods varying from 1 hr to 1 week. 



The results have shown that there is no 

 significant difference between survival times at 

 34 C of the B/K and K/B hybrids. However, 

 both in the 14.5C and 25°C adapted groups 

 the hybrids survived longer than either of the 

 inbred lines, this being particularly noticable 

 in the 14.5°C adapted groups. Of the inbreds the 

 K males were slightly more "heat-tolerant" 

 than the B males. All flies gained adaptation at 

 25 C C from 14.5°C much more rapidly than they 

 lost this adaptation when taken from 25°C and 

 put at 14.5C. The hybrids both lost and gained 

 acclimatization to 25 D C more quickly than either 

 of the inbred lines. 



It is concluded from these results that in- 

 breeding leads to a loss of biochemical versatility 

 and also to a loss of fitness. 



1. J. B. S. Haldane, (1948) Ric. sci. Supp. 54. 



2. J. Maynard Smith, (1956) /. Genet. 54, 261 . 



9.43. Life Cycle and Mating Activity as Criteria of 

 Heterosis in Heterokaryotypes in Drosophila 

 pavani. Danko Bracic and Susi Koref- 

 Santibanez (Santiago, Chile). 



The study of chromosomal polymorphism due 

 to inversions, found in many species of the genus 

 Drosophila, has proven that these constitute an 

 adaptive character, and that frequently the hete- 

 rozygotes for different gene arrangements (hete- 

 rokaryotypes) are heterotic. Nevertheless, more 

 experimental data are needed in order to deter- 

 mine the exact mechanisms by which such 

 adaptive superiority is achieved, and why 

 balanced polymorphism is maintained in nature. 



157 



