Changes in Populations 145 



Fig. 7.9 I Frequency of Standard-type chromosomes ( in percent ) in 

 different months in a single population cage. (From Dohzhansky, 1947, 

 Evolution 1 . ) 



Mar Apr May June July Aug Sep Oct Nov Dec 



D. mclanogaster, a characteristic known to be of considerable selec- 

 tive importance in laboratory populations. Among other things, 

 temperature, humidity, moisture content of the medium, larval 

 density, and length of larval period all affect the place in which the 

 larva chooses to form the pupa. The ecogenetics of Drosophila will 

 supply a fertile field for research for a long time to come. 



Examples from Man 



As one might expect, natural selection plays an active part in shap- 

 ing the genetics of populations of our own species. An outstanding 

 example of this is the selective control of the frequency of the so- 

 called "sickle-cell" gene. Individuals who are homozygous for this 

 recessive gene show distortion of their erythrocytes, accompanied 

 by severe anemia and general, serious and painful disabihty. The 

 condition is usually fatal. Heterozygous individuals may be detected 

 by the distortion (sickling) of their red blood cells, which occurs 

 when the oxygen concentration of the blood is reduced. Sickle-cell 

 heterozygotes, however, apparently are protected to some degree 

 against malaria and thus are favored by selection in malarial areas. 

 Allison found that the frequency of sickle-cell heterozygotes was 



