102 Natural Selection 



1. SICKLE-CELL 



In certain African Negro populations the allele for the blood cell 

 condition known as sickle-cell disease was found to be common. 

 This disease is the product of a gene which causes the blood cells 

 to assume a sickle-like shape in a medium deficient in oxygen. In 

 the heterozygous state this gene causes at most a mild anemia but 

 in the homozygous state causes a lethal anemia. Why sickle-cell 

 persisted in these populations was a mystery until it was discovered 

 that individuals carrying the sickle-cell allele were more resistant 

 to malaria than those with normal alleles. Therefore, the individuals 

 homozygous for sickle-cell were depleted by the pathologic blood 

 condition, and those homozygous for normal blood cells were 

 decimated by malaria. Heterozygous individuals survived best, and 

 therefore natural selection favored carriers of this genetic makeup 

 (Allison, 1955). 



2. CHROMOSOMAL INVERSIONS AND CLIMATIC ADJUSTMENT 



Another simple example in the operation of selection involves in- 

 versions in the chromosomes of the fruit-fly Drosophila pseudo- 

 obscura (Dobzhansky, 1955). Natural populations of these flies 

 sampled at different elevations in the Sierra Nevada of California 

 each had a distinctive proportion of several inversions in its genetic 

 pool. Laboratory tests showed that individuals heterozygous for 

 these inversions possessed sufficient heterosis or hybrid vigor to be 

 more successful than either homozygote. Further, each combination 

 of inversion heterozygote was most effective within particular tem- 

 peratures. Thus each elevation had a different range of temperatures, 

 and this in turn brought about the selection of that proportion of 

 the different inversions most successful within that particular tem- 

 perature range (Fig. 41). 



3. PROTECTIVE COLORATION AND MIMICRY 



The potential selective value of these has long been established 

 by experiment. Especially startling examples include the many 

 patterns of edible Cerambycidae beetles (Fig. 42) which mimic 

 patterns of stinging wasps or distasteful Lycidae beetles occurring 

 in the same habitats (Darhngton, 1938; Linsley, 1959) and the 

 protection given to various cryptically colored grasshoppers against 

 predation by birds (Isely, 1938). The realized value of such selec- 

 tion is afforded by two cases involving populations of a species of 

 snake and a species of moth, respectively. 



