690 



ECOLOGY AND EVOLUTION 



mutation rather than gene frequency is 

 probably more important in the origin of 

 higher taxonomic categories. 



Darwin, in developing his theory of in- 

 dividual sexual selection, did not always 

 recognize that the male-female pair could 

 be a unit upon which selection could act as 

 a whole. At certain levels of integration 

 there may be an evolution of sterility. This 

 is exempbfied at the level of the organism 

 by the somatic cells and at the population 

 level by the sterile castes of social insects. 

 It would be wholly illogical (see p. 599) 

 to interpret the evolution of sterility by 

 means of the survival of unit somatic cells 

 in the organism, or the survival of sexless 

 individuals in the society. 



In colonies of honeybees, drones are 

 usually produced at the time of queen pro- 

 duction when they may function in fertiliz- 

 ing new queens. Queens lay unfertilized 

 eggs in drone cells made with somewhat 

 thicker walls than cells in which workers 

 are raised (p. 434). The various behavior 

 mechanisms that enable the workers and 

 the queens to regulate the numbers of 

 queens, drones, and workers in the colony 

 at various seasons and under various con- 

 ditions are almost entirely unknown, but 

 there is no doubt that a remarkable popu- 

 lation balance is maintained. Only a few 

 drones copulate with the few young queens. 

 The others have no function in the colony 

 and are disposed of in the most direct way 

 by the workers. This population control is 

 most dramatically illustrated at the time of 

 drone elimination, which usually occurs in 

 the fall when the nectar flow diminishes. 

 Workers appear to herd the drones into 

 corners of the colony and cut them oflF 

 from a food supply. The weakened drones 

 are ultimately dragged out and left to die 

 outside the hive entrance. In one instances 

 on observation hive had the exit closed at 

 the time of the drone slaughter. The 

 workers cut up the bodies of the drones and 

 dropped the remains through a crack in the 

 bottom of the hive so that a conspicuous 

 mound of drone parts accumulated directly 

 below the hive. Queenless colonies may 

 tolerate drones indefinitely.* This func- 

 tional behavior for population control in 

 the honevbee is characteristic of the species, 

 and thus is probably the product of adap- 

 tive evolution. The regulation of numbers 



• Personal communication from O. W. Park. 



in a colony of social insects is analogous to 

 the regulation of numbers of cells and cell 

 types in a multicellular organism. 



The action of natural selection on the 

 population or organismic unit as a whole 

 does not preclude independent selection of 

 parts of higher systems. Through inde- 

 pendent assortment, chromosomes may be 

 selected independently of other chromo- 

 somes in the cells, and through crossover 

 mechanisms, genes within a chromosome 

 may be independently selected in relation 

 to other genes in the system. At other 

 levels of integration we may also expect to 

 find a degree of difiFerential selection of 

 parts. This would seem to be illustrated by 

 the evolution of secondary sexual charac- 

 ters that function during competition for 

 mates between members of the same sex 

 (intrasextial selection). 



Intrasexual selection, together with selec- 

 tion of sex pairs, may be involved in the 

 evolution of certain types of sexual be- 

 havior. Wheeler (1928, pp. 145-165) re- 

 viewed numerous cases of insect and spider 

 courtship that indicate a relationship be- 

 tween the food and sex drives. The food 

 drive originates as an individual adapta- 

 tion, while the sex drive is a population 

 adaptation. A behavior pattern functioning 

 for the acquisition of food may be in part 

 transformed by evolutionary processes into 

 acquisition of mates. 



Instances are known in which (1) the 

 males regurgitate food droplets that are 

 imbibed by the female before or during 

 copulation (CardiacephaJa mijrmex of the 

 dipterous family Micropezidae); (2) the 

 males secrete a glandular substance upon 

 which the females feed before, during, or 

 after copulation (the tree cricket Oecan- 

 thus niveus); (3) the male is devoured bv 

 the female after copulation (the mantid, 

 Mantis religiosa, and many spiders, includ- 

 ing Pisaura mirahilis); (4) the male cap- 

 tures and paralyzes its prey and presents it 

 to the female, who devours it during copu- 

 lation (several species of the dipterous 

 family Empididae, including Empis bore- 

 alis); (5) the male presents the female 

 with prey or a substitute object such as a 

 flower part which she "plays with" during 

 copulation (various species of the empidid 

 genus Hilara); and (6) the male catches 

 and kills a minute insect and encloses it in 

 an envelope made of an oral secretion that 



