NATURAL SELECTION 



645 



two species do not overlap in breeding 

 range or breeding season. The weight of 

 the individuals of the two species conforms 

 to Bergmann's rule (p. 119), the emperor 

 being markedly larger, while the bills, 

 wings, and feet remain proportionately 

 small, thus also conforming to Allen's rule 

 (pp. 119, 626). The emperor weighs from 

 57 to 94 pounds (average 70.5), while the 

 king averages about 44 pounds. The ap- 

 pendages of the two species are of nearly 

 the same absolute size. For instance, the 

 wings average 328 mm. in the king and 326 

 mm. in the emperor, while the bill and foot 

 are a little shorter in the emperor than in 

 the king. 



Most of the adaptations of the emperor 

 penguin are also found in its more northern 

 congener. One would thus conclude that 

 the major adjustments for the rigorous ant- 

 arctic environment had evolved in the less 

 extreme conditions found in the circum- 

 polar islands in the latitude of South 

 Georgia, the Falkland Islands, and the 

 Magellanic region. Having evolved inde- 

 pendence of nesting materials and rocky 

 terrain, the bird was able to invade the 

 Antarctic continental shelf ice and survive 

 there in the more extreme conditions. In 

 large part, therefore, the emperor penguin 

 was preadapted to its present rigorous hab- 

 itat, though the total adaptation is both 

 great and complex. Obviously not all the 

 adaptations arose with the origin of this 

 particular species, nor with the origin of 

 the genus, but antedate the later special 

 adjustments to special environments. On 

 the other hand, the distinctive black head 

 and contrasting white areas of the emperor 

 mav function for intraspecies recognition, 

 and the white feathers with enclosed air 

 may assist insulation. Such distinctive char- 

 acters may have been selected after the iso- 

 lation from the ancestors of the king pen- 

 guin. 



Some advocates of preadaptation are in- 

 clined to explain all functional response as 

 resulting from habitat (organic) selection 

 or from fortunate accidents of survival dur- 

 ing dispersal (see Parr, 1926). Preadapta- 

 tion helps to explain some instances of en- 

 \1ronmental adjustment, but there is dan- 

 ger of overemphasis. 



Individual choice of habitat through con- 

 ditioned behavior may be illustrated by the 

 records of the return of banded birds to 



their nesting sites of previous years, some- 

 times from a distant winter range. A male 

 bank swallow {Riparia r. riparia) is re- 

 corded (Stoner and Stoner, 1942) as nest- 

 ing within a few feet of the spot in the 

 colony where it had nested during three 

 previous recorded seasons, once with the 

 same and twice with diflFerent females. This 

 species winters in northern South America. 

 Such an exact nonrandom choice of nest- 

 ing site must involve individual learning 

 and memory as well as the species behavior 

 of all bank swallows, which commonly nest 

 in the rather restricted habitat of sand 

 banks. Natural selection of useful changes 

 caused by many random mutations may be 

 distinguished from habitat selection, in 

 which the organism actively moves into 

 new conditions (Baldwin, 1896; Conklin, 

 1943; Thorpe, 1945) . 



Instinctive or hereditary orientation may 

 also lead to a selection of the microhabitat. 

 Termite nests built by workers of Con- 

 strictotermes cavifrons in the Guiana rain 

 forest are found on the sides of smooth- 

 barked trees (Fig. 235). The nest usually 

 hangs from the under side of slanting trees, 

 and it is characteristic of the species to 

 build chevron-shaped solid ridges over and 

 above the nest on the tree trunk. These 

 ridges are beautifully adapted to deflect 

 water descending the trunk and doubtless 

 represent the consequence of selection of 

 hereditary behavior patterns in these social 

 insects over long periods of time. However, 

 the colony is established by a roval pair 

 that flies from the parental nest and. after 

 shedding the wings, digs a shelter in the 

 soil or in a decaying log. The workers ulti- 

 mately developing from the eggs of the 

 queen then choose the site for the nest, 

 build at least a portion, and induce the 

 royal pair to migrate to the new site which 

 would have been unsuitable for the initial 

 establishment of the colony (Emerson. 

 1938). 



This is an example of adaptation to a 

 complex environment with restriction to 

 parts of the whole habitat during portions 

 of the individual and population life cycle, 

 succeeded by movement to chosen sites at 

 other stages of the life cycle. Organic selec- 

 tion or choice of habitat may be automatic 

 and not intelligent, reasoned, or conditioned 

 (Thorpe, 1945), but it nevertheless in- 

 volves complex behavior. 



