Section 9 — Population Genetics 



very quickly in the succeeding generations to 

 almost zero in generation 15. The drastic 

 elimination of such lethals (14 in total, in two 

 different experiments) after so few generations 

 could be explained by changes, due mainly to 

 environmental factors, in the adaptative values 

 of the zygotic combinations carrying lethals. 



If this hypothesis were correct, a change in the 

 frequencies of chromosomes bearing lethals in a 

 control island would be expected. We have four 

 samples analysed from a control island, Quei- 

 mada Grande, very close to the other two. The 

 samples were taken in June and July when the 

 frequencies of lethals should be high and in 

 October and January when the frequencies should 

 be lower. The frequencies found were: 0.41, 

 0.39, 0.39 and 0.34 per cent respectively. No 

 statistically significant difference was found. This 

 shows that there are no corresponding increases 

 of elimination of lethal chromosomes, in the 

 natural population for their own lethal, present 

 in very low frequencies. 



This comparison suggest, that the relative fre- 

 quencies of the genetic components could be very 

 important in their own dynamics. 



9.47. A New Type of Hybrid Sterility in Drosophila 

 paulistorum. L. Ehrman (New York City, 

 U.S.A.). 



It has been shown that Drosophila paulistorum 

 is a group of at least six races or incipient species, 

 and that, at our time level, reproductive isolation 

 is in the process of formation between these 

 incipient species. Crosses between three (Centro- 

 American, Amazonian, Andean-South Brazilian) 

 of the six races result in the production of fertile 

 female and sterile male hybrids. The male sterility 

 has been found to depend on the genotype of the 

 male's mother. Any female which carries any 

 mixture of the chromosomes of different races 

 deposits eggs giving rise to sterile male zygotes 

 and to fertile female ones. The male sterility is in- 

 dependent of the genotype of the male parent and 

 the genotype of the sons themselves. What hap- 

 pens is evidently that the presence of a foreign 

 chromosome in the female so alters the structure 

 of her eggs, presumably by some modification 

 of the egg cytoplasm, that male individuals devel- 

 oping from these eggs are sterile, and this 

 regardless of the chromosomal complementwhich 

 they come to possess after fertilization. Further- 

 more, any one foreign chromosome (the species 

 has three pairs of chromosomes) suffices to in- 

 duce this male sterility. 



More recent work involving all six of the D. 

 paulistorum races permits the categorizing of six 



distinctly different manifestations of this new 

 type of hybrid sterility; these further emphasize 

 the unique dichotomy in the genetic causation of 

 the Fi and of the backcross hybrid male sterility 

 exhibited by this species complex. 



9.48. Subspeciation in Drosophila melanogaster. 



G. Eloff (Bloemfontein, South Africa). 



Cultures were made of more than 30 popula- 

 tions of Drosophila melanogaster collected over 

 Southern Africa at localities, some of which are 

 almost 1000 miles distant and involving im- 

 portant ecological differences. These populations 

 were bred under similar conditions in the labo- 

 ratories of the Dept. of Genetics of the Orange 

 Free State (20°C; very dry conditions; banana, 

 agar and yeast moldex food recipe). Of each 

 population the average yield per 10 females was 

 determined. Great differences in number of off- 

 spring were observed. These different geographic 

 populations were intercrossed and the number of 

 Fi per 10 females of each cross counted. The 

 number of offspring ranges from nil to more 

 than the average for each parental population. 



Furthermore, it was found that in some cases 

 a population would not cross with Oregon River 

 or Florida population of Drosophila melanogaster 

 but would cross with another geographic race, 

 which inks turn does cross with Oregon River. 

 Several of these transitional stages were recorded. 



These three aspects of geographic populations, 

 viz. (1) differences in fitness under similar con- 

 ditions of breeding; (2) differences in average 

 number of Fi of the different interpopulation 

 crosses; and (3) degrees of succesful crossing 

 between the different geographic populations, 

 strongly suggest considerable genetic differen- 

 tiation and perhaps subspeciation in progress. 



9.49. Isolation by Disruptive Selection. J. B. Gibson 

 and J. M. Thoday (Cambridge, Great Britain). 



Thoday and Gibson* 1 ) exposed a population 

 "Southacre" of Drosophila melanogaster to 

 disruptive selection for sternoplural chaeta 

 number using a breeding system that ensured 

 that all selected flies were together in a single 

 3 x 1 in. vessel for the limited period during which 

 all mating had to occur. Their population rapidly 

 split into two components between which little 

 if any effective mating occurred. 



Selection was discarded in this line when ex- 

 treme fertility trouble occurred at generation 28. 

 However, isolated high and low chaeta number 

 lines were maintained. A new line was started 



159 



