628 



ECOLOGY AND EVOLUTION 



the secondary result of gene action selected 

 on the basis of other functional adjustments 

 of the animal. Experiments by Dice (p. 

 650), however, indicate predation as the 

 probable selective factor in a similar case. 



Ecotypes may differ in physiological 

 characters rather than in visible ones. Clau- 

 sen, Keck, and Hiesey (1940) experimen- 

 tally studied the tolerance of different eco- 

 types of the plant Potentilla glandulosa in 

 different habitats and found that plants 

 produced from seeds from other and dif- 

 ferent climatic zones were unable to sur- 

 vive as well as plants from seeds produced 

 locally. 



Biotic conditions rather than physical 

 factors may sometimes bring about a selec- 

 tive pressure that produces physiologically 

 adapted races. Parasitic species dependent 

 upon dispersal through vectors commonly 

 have adjustments to the behavior as well as 

 to the physiology of their hosts. Larval 

 microfilariae of the nematode, Wuchereria 

 bancrofti, usually exhibit nocturnal periodic- 

 ity in the human blood stream in associa- 

 tion with the night-biting mosquito that 

 transmits them. A nonperiodic race or spe- 

 cies of this parasite, morphologically simi- 

 lar to the periodic race, is carried by a day- 

 biting mosquito, Aedes varies.atus, the geo- 

 graphic distribution of which is closely as- 

 sociated with the distribution of the non- 

 periodic filaria in the islands of the Pacific 

 (Manson-Bahr, 1940, pp. 750, 950). If in- 

 tergradations occur between such popula- 

 tions, ecotypes would be found based upon 

 biotic factors of the environment. 



Obviously hereditary variation in asexual 

 species is based wholly upon mutations and 

 not upon reassortment and combination, 

 which can take place only through fertiliza- 

 tion. Asexual species have either evolved 

 from ancestors in which sex never devel- 

 oped, or they may have arisen from sexual 

 species by means of complete partheno- 

 genesis (White, 1945, p. 280). Reproduc- 

 tive isolation exists between asexual individ- 

 uals after multiplication, and no muta- 

 tion can pass to a new individual except 

 through clonal descent. The continuance of 

 a mutation must be largely through selec- 

 tion of the functions it affects (Lewis, 

 1934; Parr, 1938). A number of functional 

 characteristics may be initiated by a single 

 gene, so that it is possible that a mildly 

 harmful effect may be overbalanced by a 



stronger beneficial effect. Neutral charac- 

 ters also doubtless persist. 



Certain types of adaptation are more 

 likely to appear and persist in asexual spe- 

 cies than in sexual species. For example, 

 there may be speciation in a parasitic form 

 without other types of isolation, and it 

 may be associated with selection toward 

 adaptation to different parts of the same 

 host body. In a sexual species, reproductive 

 isolation would probably not be complete 

 enough to avoid swamping any new incip- 

 ient adaptation in such a narrow ecologic 

 niche. It is true that we do know some 

 cases of sexual species of parasites living 

 on different parts of the host, but it seems 

 probable that other types of isolation than 

 habitat isolation within microniches on the 

 same animal or plant may have operated 

 (Mayr, 1942, p. 204). 



Wenrich (1944, 1944a) reports Tricho- 

 monas hominis in the intestine of man, T. 

 tenax in the mouth, and T. vaginalis in the 

 vagina. These species are physiologically as 

 well as morphologically distinct and do not 

 survive experimental transplantation from 

 one body region to another. However, T. 

 hominis has also been found in monkeys, 

 cats, dogs, and rats, and can be experimen- 

 tally transferred to the intestines of these 

 animals. This genus of protozoans, together 

 with other related genera, is not known to 

 conjugate. 



Protozoa of the genus Leishmania, patho- 

 genic in man, produce two distinct 

 diseases: the oriental sore or cutaneous 

 leishmaniasis is caused by L. tropica, and 

 kala-azar or visceral leishmaniasis is caused 

 by L. donovani. The two species of proto- 

 zoans are indistinguishable morphologically, 

 but affect different regions of the body in 

 both man and dogs (Hoare, 1943; Kirk, 

 1944). 



Similar cases of such adjustment to mi- 

 croniches within the same host are numer- 

 ous among pathogenic bacteria in which 

 strains seem to evolve rapidly with subtle 

 but distinctive pathological symptoms 

 (Lewis, 1934; see also p. 601). 



Asexual parasitic species could also easily 

 become separated as they became selected 

 for different hosts. Hoare (1940) has 

 shown that Trypanosoma evansi, a widely 

 distributed protozoan infecting domestic 

 ungulates and other mammals, is not phys- 

 iologically adjusted to the tsete fly (Glos- 



