NATURAL SELECTION 



689 



adaptations for combat between males. Re- 

 cently, various authors have been incUned 

 to treat sexual adaptation and other types 

 of group adjustment as similar in basic 

 origin to endoadaptation within the organ- 

 ism. 



Many groups of invertebrates, including 

 flatworms, oligochaetes, leeches, and most 

 moUusks, are hermaphroditic. Other groups 

 such as echinoderms, arthropods, and 

 vertebrates are with few exceptions bi- 

 sexual. Bisexuality has convergently evolved 

 from hermaphroditism many times, and 

 hermaphroditism has also arisen through 

 bisexual forms a number of times (White, 

 1945, p. 228). It is obvious that sexual tis- 

 sues (male or female) were originally with- 

 out genetic diflFerentiation, that in some bi- 

 sexual forms there is no genetic determina- 

 tion, and that it was in the later stages of 

 the evolution of sex that genetic determina- 

 tion arose, and is particularly well de- 

 veloped in the insects and vetebrates. 



The intricate interplay of sexual, familial, 

 and species mechanisms of attraction indi- 

 cates various levels of group coordination 

 resulting from identical physiological or be- 

 havior adaptations (Noble and Curtis, 

 1939). Selection sorts more efficient mech- 

 anisms through survival of the whole sys- 

 tem. In sexual mechanisms the survival 

 unit is the sex pair, and not the individual 

 as such. Marshall (1942) points out that 

 display and courtship often occur after and 

 not before the period when the birds are 

 paired, so that courtship is subsequent to 

 choosing a mate. 



Display is not always confined to one 

 sex, but is often used for mutual stimula- 

 tion. Communal display by large numbers 

 of individuals may be useful in stimulating 

 each individual female, even though many 

 of the males at the bottom of the peck 

 order may not copulate (J. W. Scott, 

 1942). 



Predators may produce a selective pres- 

 sure favoring concealing coloration instead 

 of conspicuous sexual coloration, partic- 

 ularly in the brooding female. Bright- 

 colored females are often found in hole- 

 nesting birds and in female-courting; spe- 

 cies such as the phalaropes (Huxlev, 

 1938). The male phalarope is dull-colored, 

 builds the nest, incubates the eggs, and 

 tends the young. 



Newts (Triturus spp.) exhibit courtship 



attitudes after the males have dropped 

 their spermatophores. If the males do not 

 perform, the females do not pick up the 

 spermatophores. Huxley (1941) points out 

 that the female cannot know that a particu- 

 lar spermatophore has been dropped by a 

 particular male, so that Darwinian sexual 

 selection is hardly possible in this instance. 



It should be emphasized that other phys- 

 iological attributes of sex do not function 

 in sex attraction and can hardly be selected 

 in the Darwinian sense of sexual selection. 

 For example, female sex hormones stimu- 

 late the development of mammary glands 

 in mammals. These glands are clearly an 

 adaptation of the mother for the benefit of 

 the young, and thus help to coordinate the 

 family. They can hardly be assumed to 

 evolve through the selection of females by 

 males that would choose more e£Fective 

 mammary glands, nor can the offspring 

 select its mother, but selection may well 

 operate on the family unit as a whole to- 

 ward the evolution of efficient parental 

 care. 



Many angiosperm flowers are adapted 

 for display (p. 249), not to attract an in- 

 dividual of the opposite sex, but to attract 

 the animals that transfer the pollen to an- 

 other flower, thus setting the stage for ulti- 

 mate union of the gametes. There is thus 

 a sexual function and a sexual display, but 

 obviously no such mechanism could evolve 

 through individual sexual selection. The 

 species population as well as its compo- 

 nent individuals constitutes the unit of se- 

 lection in the evolution of flowers. (Inter- 

 species populations are also units of selec- 

 tion; see p. 698.) Lewis (1942) assembles 

 evidence pointing to the evolution of die- 

 cious organisms from hermaphrodites or 

 monoclinous types, usually with a mone- 

 cious intermediary. Animals are usually 

 diecious— an adjustment better suited to 

 their motility. Plants are usually hermaph- 

 roditic with adaptations for cross fertili- 

 zation—an adjustment better suited to 

 their sessile life. Both gain greater evolu- 

 tionary potentialities through reassortment 

 of chromosomes in sexual union. Greater 

 variability is an important basis for adap- 

 tive evolution, provided it is not too dras- 

 tic (Mather, 1943). Whereas reassortment 

 of existing; genes is probably a more impor- 

 tant mechanism in the origin of species 

 than is gene mutation (pp. 600, 641), gene 



