ECOLOGY AND ISOLATION 



629 



sina) . The tsetse fly is the vector for the 

 morphologically similar T. brucei, T. rho- 

 desiense, and T. gambiense in Africa. The 

 natural insect vectors for T. evansi are 

 horseflies ( Tabanidae ) . The ranges of tsetse 

 fly-borne trypanosomes and tabanid-borne 

 trypanosomes overlap in Abyssinia and the 

 Sudan. Trypanosoma evansi may have 

 arisen from T. brucei or a species close to 

 it, and through selection may have become 

 adapted to different hosts; it is now found 

 in southern Asia, the islands of the Pacific 

 and Indian Oceans, Asia Minor, south- 

 eastern Europe, the Mediterranean coast of 

 North Africa, and in some parts of West 

 Africa. Strains identical with or closely re- 

 lated to T. evansi are also found in Central 

 and South America. The theory of the ori- 

 gin of this species through natural selection 

 is supported by experiments that show that 

 any strain of the brucei group, when kept 

 out of contact with tsetse flies for a long 

 time through mechanical transmission, is 

 likely to lose its ability to develop cychcally 

 in Glossina. 



Asexual organisms probably are more 

 efficient in reproductive capacity, other 

 things being equal, than are sexual types, 

 which must face the exigencies of success- 

 ful gamete attraction and fertilization. An 

 asexual organism does not have the advan- 

 tage of gene recombination that supplies a 

 large foundation of heritable variation sub- 

 ject to selective pressures (White, 1945, 

 p. 281; see also p. 641). However, muta- 

 tions are more easily fixed in the asexual 

 population, and advantageous genes are 

 thus more likely to be selected. Neutral 

 variations are also likely to be perpetuated 

 over long periods of time. 



Cleveland, Hall, Sanders, and Collier 

 (1934) and Cleveland (1947) have de- 

 scribed four species of flagellates of the 

 genus Barbulanympha from the intestines of 

 the roach, Cryptocercus punctulatus, all four 

 species being found in the separated popu- 

 lations of the host in the Pacific and Ap- 

 palachian regions. The flagellate species 

 differ from each other in body size, number 

 of organelles, and in numbers of chromo- 

 somes that seem to be multiples of eight 

 (numbers 16, 32, 40, and 48 to 52 are re- 

 corded). There is no information on popu- 

 lation numbers or possible ecologic differ- 

 ences, but there is no reason for assuming 

 any exoadaptive divergence during specia- 



tion. Inasmuch as these asexual Protozoa 

 are found exclusively in the intestines of 

 one sexually reproducing insect species, it 

 may be assumed that speciation in the Pro- 

 tozoa was more rapid than speciation in 

 the host. Other genera of flagellates also 

 seem to have speciated in the intestine of 

 this one host. Trichonympha (sexual fertili- 

 zation) has seven species, Leptospironym- 

 pha (sexual fertilization) has three species 

 (two in the Pacific region, one in the Ap- 

 palachian region), and Saccinobaculus (au- 

 togamous) has three species. 



Although some asexual Protozoa in 

 Cryptocercus seem to have evolved more 

 rapidly than their sexually reproducing 

 hosts, both flagellates and hosts are remark- 

 ably stable. Cryptocercus punctulatus shows 

 an interesting discontinuous distribution in 

 the Appalachian mountains and in the 

 Pacific Northwest. Eighteen species of Pro- 

 tozoa are found in both populations, while 

 four species are found in the western hosts 

 alone and three species in the eastern hosts 

 alone. The distribution of the roach is 

 limited by its reliance on moist logs for pro- 

 tection against freezing temperatures, with 

 relatively cool conditions in summer. The 

 cellulose-digesting Protozoa have been 

 shown to die if a temperature of 30° C. is 

 maintained for twenty-four hours. The 

 western and eastern ranges of the roach 

 could not have been continuous later than 

 the close of the last glacial period. Hence 

 we have a measure of the evolutionary 

 stability of the roach and many species of 

 Protozoa. Protozoa from the intestines of 

 termites often show a similar tendency to- 

 ward more rapid speciation than do their 

 hosts (Kirby, 1937, 1942a). Other termite 

 protozoans evolve more slowly than their 

 hosts. 



Some authors (Dobzhansky, 1941, p. 

 378) would ehminate asexual taxonomic 

 groups from the species category and define 

 species as interbreeding populations. In 

 some asexual groups, such as certain bac- 

 teria and the plant genus Hieracium, muta- 

 tion is so frequent that species designations 

 become impractical. In other asexual 

 groups, such as the flagellates in the intes- 

 tines of termites, species distinctions remain 

 stable for long periods of time. The essen- 

 tial difference between asexual and sexual 

 species is the complete reproductive isola- 

 tion of asexual individuals in the same 



