THE ORIGIN OF SPECIES 







_,*^ \A — K -"* 









P''%S-##>" ^^^t*#^"' 



,;<-;v 



Figure 89. See opposite page for legend. 



vides a source of variability which permits a species to adapt rapidly to 

 environmental changes. Most natural populations of sexually reproducing 

 organisms are highly heterozygous. Whenever this heterozygosity results 

 in obviously different phenotypes, one speaks of polymorphism. The most 

 striking cases are those of mimetic polymorphism (see below). Specific 

 selective forces may maintain a balanced polymorphism. The inversion 

 types of Drosophila pseudoohscura were introduced above. Dobzhansky 

 has shown that some chromosomal arrangements increase in frequency 

 during the summer, others during the winter, hence they must be related 

 to seasonal adaptations. A particularly instructive case is known in man. 

 Sickle cell anemia is a fatal disease resulting from homozygosity of a cer- 

 tain gene. The same gene, when heterozygous, causes the harmless sickle 

 cell trait. Strong selection pressure should tend to eliminate such a gene, 



2S8 



