Section 9 — Population Genetics 



tude and organization of the genetic load (Band 

 and Ives, 1963). Experiments by Band (1963) 

 indicate that if the gene pool remains coadapted, 

 then genetic changes which occur must be 

 accompanied by the maintenance of develop- 

 mental homeostasis among the random hetero- 

 zygotes. 



Trends toward more adverse environmental 

 conditions have been observed during 1961 and 

 1962. The lethal allelism frequency of 1 per cent 

 in 1961 indicates reduction in population size; 

 tests for 1962 are in progress. Genetic variance 

 (a^) relationships between random heterozygotes 

 and nondrastic (nd) homozygotes in 1960, 1961 

 and 1962 reveal changes in genetic organization. 

 In 1960 erf het < a'l „,„„, in 1961 o'l het > os\ h0m 

 due to the genetic variance of the nd/nd hete- 

 rozygotes; in 1962 a\ h et < ct| hom . Never- 

 theless, in both 1961 and 1962 heterozygotes 

 containing two, one or no "drastic" second 

 chromosomes possess comparable developmental 

 homeostasis, superior to that of the nd homo- 

 zygotes in each collection. 



These findings provide evidence that the 

 genetic diversity carried in the gene pool enables 

 the population as a whole to shift from one of 

 Wright's adaptive peaks to another. They also 

 indicate that the attainment of new equilibrium 

 gene frequencies is influenced by a variety of 

 factors both external (environmental) and in- 

 ternal (the achievement and maintenance of 

 developmental homeostasis; Band, 1963). Thus, 

 selection for the maintenance of developmental 

 homeostasis may lead to the retention of geno- 

 typic variability in the population (Lerner, 

 1956). 



Supported by grants from the National Science 

 Foundation and the National Research Council 

 of Canada. 



9.39, The Estimation of Population Fitness. J. A. 



Beardmore and W. van Delden (Gronin- 

 gen, The Netherlands). 



In the simplest terms the fitness of a popula- 

 tion is the weighted average of the fitness of the 

 individual genotypes of which it is composed. 

 Two practical difficulties encountered in connec- 

 tion with this simple statement are: first that it 

 is almost impossible in outbreeders to specify all 

 the genotypes concerned (thus preference is 

 given to polymorphisms as objects for study) 

 and secondly even when the genotypes are 

 specified the estimation of the fitness of indivi- 



dual genotypes may have to be made in isolation 

 and hence interactions of all types are ignored. 

 This leads to a situation where empirical obser- 

 vations of suitable populations may be valuable 

 in assessing ielative population fitness over both 

 short and long periods. We have attempted to 

 estimate the relative fitness of a number of labo- 

 ratory populations using as indices of fitness 

 biomass production, longevity, fertility and other 

 characters. We hope to report on the correlation 

 between genetic variance and fitness in a stan- 

 dard environment, correlation of fitness values 

 in different environments and the environment 

 as a component of fitness. 



9.40. Synchronous Mutability Changes in Geogra- 

 phically Isolated Populations of Drosophila 

 melanogaster. R. L. Berg (Leningrad, 

 U.S.S.R.). 



During 1937-41 high concentrations of yel- 

 low mutants and high mutation rates of the 

 yellow locus were detected in five geographi- 

 cally isolated populations of Drosophila me- 

 lanogaster. The studied populations were Uman, 

 Ukraine (1937), Nikita Botanical Gardens, 

 Crimea (1937-38), Dilijan and Erevan, Armenia 

 (1939), Kashira, near Moscow (1940-41). 



During 1945-46 we studied the populations of 

 Kutaissi, Georgia (1945), Tiraspol, Moldavia 

 (1946) and Uman (1946). In Uman and Tiraspol 

 high concentrations of yellow mutants were 

 again observed. The rate of occurrence of the 

 yellow mutation proved to be lower than during 

 the previous period of investigations. 



During 1957-62 low concentrations of yellow 

 mutants and low mutation rates were seen in 

 nine populations (Piatigorsk and Inosenitsevo, 

 North Caucasus 1957; Uman, 1958, 1962; 

 Nikita Botanical Gardens, 1960, 1962; Dilijan 

 1960-62; Kashira, 1960, 1962; Alma-Ata, 1961; 

 Tiraspol 1962). 



The concentration of yellow mutants among 

 wild males was: in Uman in 1937 0.246 ± 0.49 

 per cent (25 yellow males among 10,159 wild 

 males), in 1946 0.301 ± 0.090 (11 yellow males 

 among 3652 males), in 1958 there were no yel- 

 low mutants among 13,131 males. 



The mutability in the Uman population in 

 1937 was 0.149 ± 0.027 (30 yellow males among 

 20,144 males) in 1946 in a mutability test among 

 5394 males we found no yellow mutants and in 

 1958 among 84,831 males in the progeny of wild 

 flies, only three (0.035 ± 0.020 per cent), i.e. 

 one forty-third, of the percentage observed in 

 1937. 



156 



