686 



ECOLOGY AND EVOLUTION 



changes in form exhibited by entomostra- 

 cans (Crustacea) (Fig. 21). The term 

 may be appropriately applied to all popula- 

 tions exhibiting periodic polymorphism. 

 Examples include malarial protozoans (p. 

 701), flukes (Fig. 249), tapeworms (Fig. 

 250), and aphids (pp. 123, 612, 703). 

 Cyclomorphic species exhibit a life cycle of 

 a population and not just of an individual 

 metamorphic organism (Huff and Coulston, 

 1946). 



Many generations may occur in a year 

 among Cladocera adapted to flotation, tur- 

 bulence, and locomotion near the surface of 

 fresh water (Coker, 1939). Density and vis- 

 cosity of the water change with temperature. 

 Pure liquid water is most dense at 4° C. 

 and is less dense above and below this tem- 

 perature (p. 93). Winter forms need less 

 adjustment to flotation and motion than 

 summer forms, and certain species of Clado- 

 cera (Bosmina) undergo seasonal changes 

 in shape, offering more surface to the less 

 dense and less viscous warmer water and 

 less surface to the more viscous colder 

 water. Certain plankton protozoa {Cera- 

 tium), rotifers (Asplanchna) , and diatoms 

 likewise exhibit seasonal cyclomorphic 

 changes in form. 



It is obvious that the genetic constitu- 

 tion may be identical in these distinct 

 phases of the population life cycle. Repro- 

 duction is asexual or parthenogenetic for 

 most of the generations, so that thousands 

 of individuals belonging to a single clone 

 carry identical heredity, while the genera- 

 tions within the clone differ strikingly in 

 their morphologic and physiologic adapta- 

 tions. 



The evolution of such cyclic polymor- 

 phism must be explained through the selec- 

 tion of a genetic system in the species as 

 a whole that responds to different stimuli 

 by differential development in different en- 

 vironments, just as different structural 

 adaptations develop in the soma of an or- 

 ganism from genetically identical cells in 

 different physiological settings. In other 

 words, the winter-adapted cladoceran must 

 not only respond to the environment of the 

 individual in January, but must have a 

 genetic pattern capable of responding dif- 

 ferently to the summer environment several 

 generations hence. It follows that the unit 

 of selection cannot be only the genetic pat- 

 tern of the individual in its habitat, but 



must be also the genetic pattern of the 

 whole species population in its seasonal en- 

 vironment (see also p. 664). 



Likewise the population of a species of 

 malarial protozoan is selected as a unit both 

 in relation to its mosquito host and to its 

 vertebrate host environment and to differ- 

 ent tissues in each (p. 701). In malarial 

 and other parasites, the physiologic adapta- 

 tion of different generations of the same 

 species is both subtle and intricate. 



A great many aggregations of the higher 

 vertebrates exhibit individual behavior dif- 

 ferences that unify the population system. 

 As with physiologic and instinctive reac- 

 tions in some types of emigrating popula- 

 tions (lemmings, grasshoppers) and in 

 cyclomorphic species, conditioned, learned, 

 and intelligent behavior is not necessarily 

 based upon genetic differences between the 

 individuals responding differently, but is 

 explained as resulting from the action of 

 selection on the whole unitary population in 

 favor of a capacity for plastic response. 

 Even a flock of inbred hens arranges itself 

 in a peck order, and such a genetic capacity 

 for conditioning might have been selected 

 during the evolution of the species (pp. 

 413, 663). The capacity for somatic adap- 

 tation certainly evolves and is an important 

 basis for the evolution of the brain capac- 

 ity and intelligence found among higher 

 vertebrates (pp. 639, 693). 



Wheeler (1928b, p. 12) hsts the stages 

 of the evolution of the insect (especially 

 hymenopteran) family and society as fol- 

 lows: 



1. The insect mother merely scatters her 

 eggs in the general environment in which 

 the individuals of her species normally live. 

 In some cases the eggs are placed near the 

 larval food. 



2. She places her eggs on some portion 

 of the environment (leaves, and the like) 

 which will serve as food for the hatching 

 larvae. 



3. She supplies her eggs with a protec- 

 tive covering. This stage may be combined 

 with (1) or (2). 



4. She remains with her eggs and young 

 larvae and protects them. 



5. She deposits her eggs in a compara- 

 tively safe or specially prepared situation 

 (nest) with a supply of food easily acces- 

 sible to the hatching young (mass provi- 

 sioning). 



