Section 9 — Population Genetics 



fer, especially with respect to the distribution of 

 variation about the mean, the evidence for gene- 

 environment interaction and also whether or not 

 development time and body size are correlated. 

 Selection for either shorter larval period or high- 

 er egg production, under novel conditions, 

 led to substantially smaller or larger size, sug- 

 gesting that the normal stability of the latter is 

 maintained by opposing selection pressures. 

 Populations adapted to different diets have 

 shown consistent changes in body size and 

 components of fitness and thus provide material 

 for the experimental study of evolutionary 

 change. Most of this work is in press and 

 will appear in the Journal of Genet ical Research. 



9.68. Disruptive and Stabilizing Selection on a 

 Mutant Character. W.Scharloo, (Leyden.The 

 Netherlands). 



Both disruptive and stabilizing selection were 

 practised on the expression of the mutant cu- 

 bitus interruptus dominant of Gloor (ci D_G ) in 

 Drosophila melanogaster. In this ci D allele the 

 relation between factors affecting expression 

 (i.e. the extent to which the fourth longitudinal 

 wing vein becomes deleted) and their phenotypic 

 effect is almost linear whereas the original ci D 

 shows a strongly nonlinear relation. 



A base population (B) was founded by intro- 

 ducing the mutant into the Pacific wild stock. 

 In the stabilizing line (S) animals with expres- 

 sion values nearest the mean were selected as 

 parents of the next generation (4 33' 4$9, among 

 20 of each sex measured). In the disruptive line 

 (D) the two 3S and $$ with highest and the 

 two $9. and 3<$ with the lowest values were 

 chosen. Each generation consisted of four bottle 

 cultures. A rotational mating system was used. 



The mean expression values did change in a 

 minor degree only, but selection was very 

 effective in both lines with respect to variances. 

 In D the phenotypic variance increased in 6 gen- 

 erations to about 2 times the value in B; in 

 S the phenotypic variance became reduced 

 to half this value. Estimated heritabilities were 

 0.50, 0.58 and 0.88, in S, B and D respectively. 



The independent variance component (based 

 upon the differences between the two wings of 

 one fly) did not change in this period. The 

 common environmental variance component 

 (non-genetic differences between flies) is of mi- 

 nor importance in this character and in the se- 

 lection response. This is corroborated by the 

 finding that there was no difference between D 

 and S in the response of expression to temper- 

 ature change. 



In generation 10 the phenotypic variance in 

 D showed a further increase to about 4 times 

 the original value, but in S there was no further 

 decrease. 



This work was begun at the Institute of Ani- 

 mal Genetics, Edinburgh, during tenure of a 

 N.A.T.O. Science Fellowship awarded to the 

 author by the Netherlands Organization for the 

 Advancement of Pure Research. 



9.69. Chromosomal Polymorphism and Position 

 Effect in Drosophila subobscura. D. Sperlich 

 (Vienna, Austria). 



Chromosomal polymorphism is common in 

 natural populations of Drosophila subobscura. 

 The frequency and the distribution of various 

 structural types are different in northern and 

 southern populations. The degree of structural 

 heterozygosity and the inversion frequency 

 are higher in Italy than in Austria and again 

 higher in Austria than in Scandinavia. Both 

 values are lower in insular isolation than on 

 the neighbouring continent. The association of 

 linked inversions is not at random although the 

 map distances between the inversions are some- 

 times large. There are regions in the chromo- 

 somes which show accumulation of breaks for nat- 

 ural inversions. An X-ray induced inversion 

 was found to be heterotic with three other 

 natural inversions tested. The distal break of this 

 inversion is identical with that of some natural 

 inversions. Therefore one may assume that 

 position effect plays a great role for the origin 

 of chromosomal polymorphism in Drosophila 

 subobscura. 



9.70. Mating Propensity of Gene Arrangement 

 Carriers in Drosophila persimilis. Eliot B. 

 Spiess and Bozena Langer (Pittsburgh, 

 U.S.A.). 



Multiple choice matings in 24 hr tests of ho- 

 mokaryotype Whitney and Klamath arrange- 

 ments of chromosome III demonstrated a rela- 

 tive propensity of 2 : 1 for W/W 33 to K/K 33 

 (Spiess and Langer, 1961). Tests were repeated 

 with varying ratio of W^: K^ in the mating 

 chamber from 9 : 1 to 1 : 9; the 3 propensity 

 varied randomly from 1.2 to 2.2 W^ P er 

 K3 mating. W/W§9_ were a ' so mated with more 

 frequently than K/K $$ except at the extremes 

 of the 3 frequency ratio when $9. mated about 



166 



