688 



ECOLOGY AND EVOLUTION 



course, may be secondary effects of the 

 genes selected through physiological fitness. 

 Another type of genetically determined 

 permanent polymorphism of a seemingly 

 functional value is described by Ford 

 (1940). Two forms of the nymphalid but- 

 terfly, Hypolimnas dubius, occur in East 

 Africa. One form is supposed to be a mimic 

 of the two species, Amauris albimaculata 



iOOi 



1930 1931 1933 



Fig. 248. The relative frequencies of black 

 and red beetles (Adalia bipunctata) in April 

 and October during three different years in the 

 vicinity of Berlin, Germany. (Redrawn from 

 Timof eeff-Ressovsky. ) 



and A. echeria. The other form supposedly 

 mimics Amauris niaviiis dominicanus. The 

 two forms differ in color, pattern, and be- 

 havior, but the different form patterns are 

 expressed through the action of a single 

 gene and its allele. Ford explains the evo- 

 lution of a dimorphic species that mimics 

 two different species on the assumption 

 that, in Batesian mimicry, selection against 

 the mimic increases as the numbers ap- 

 proach those of the model. If a mimicking 

 species can become polymorphic, a much 

 larger population can survive. 



If further study of the survival of these 

 populations in relation to each other and 

 to their predators tends to substantiate 

 Ford's speculations, we may have an ex- 

 ample of the evolution of a population ad- 

 justment based upon genetic classes in con- 

 trast with the much more common non- 

 genetic polymorphism. In spite of the single 

 gene difference between the dimorphic 

 forms of the mimic, such an evolution 

 could occur gradually and be polygenic, 

 only the threshold of expression being con- 

 trolled by the single gene. Otherwise we 

 must assume mutations that chance to re- 

 semble the model species in a variety of 

 characters with elimination of the nonmi- 



metic variations (Ford, 1936) and, in a 

 few instances, selection of genes with 

 different adaptive effects in terms of their 

 incidence in the whole population. Such 

 striking adaptation as that of mimicry 

 would be more likely to be polygenic (p. 

 670). 



Genetically determined forms may main- 

 tain their relative proportions in a popula- 

 tion with differential survival. For instance, 

 if the male sex has a higher mortaUty than 

 the female, the resulting selection would 

 not change the sex ratio in future genera- 

 tions if one sex were homozygous for the 

 sex chromosomes (XX) and the other were 

 heterozygous (XY). 



The group functions must be highly 

 adaptive, as in sexual adaptation in higher 

 organisms, before genetic determination 

 can produce classes on the basis of homozy- 

 gous and heterozygous individuals, or 

 through some other genetic mechanism. Sex 

 determination in the lower forms is com- 

 monly on the basis of physiologic differen- 

 tiation without genetic differences of the 

 sexes, and it is usually in the higher organ- 

 isms that we find genetic determination of 

 sex (the mating types of Paramecium may 

 be an exception; Sonneborn, 1939, 1941). 



In the Hymenoptera and in a few other 

 arthropods, haplodiploidy occurs— that is, 

 the males develop from unfertihzed hap- 

 loid eggs and the females from fertihzed 

 diploid ones. White (1945, p. 267) says: 

 "From the point of view of reproductive 

 economy haplodiploidy is an infinitely 

 plastic system in which (given suflBcient 

 time) selection can bring about any sex 

 ratio which is in the interest of the species. ' 

 Considering the fact that the sterile castes 

 of the social Hymenoptera are always 

 genetic females, this abihty to produce 

 large populations of females is important in 

 the convergent evolution of strictly social 

 Hymenoptera (pp. 690, 691). The termites, 

 however, evolved a social system with ster- 

 ile castes with no haplodiploidy, both sol- 

 diers and workers being sterile males and 

 females. 



Controversy has raged over Darwin's 

 theory of sexual selection through individ- 

 ual (sometimes unilateral) choice. This 

 theory, as a special aspect of natural selec- 

 tion, was thought to explain the evolution 

 of courtship displays and the evolution of 

 structural, physiological, and behavioristic 



