156 



Increase in Number of Species 



or a genus contains only a single species which is apomictic, as is the 

 sawfly genus Endelomyia. The four known apomictic (parthenoge- 

 netic) species of black flies ( Simuliidae ) occur in three genera, 

 indicating that in these insects the phenomenon occurred independ- 

 ently three and perhaps four times, with no observed subsequent 

 increase in the number of species in any one of the phylogenetic 

 lines (Basrur and Rothfels, 1959). In these cases it is highly prob- 

 able that no species splitting has occurred since the evolution of the 

 original apomictic parent. 



In other instances of apomixis subsequent splitting has occurred. 

 One of the most extensive apomictic evolutionary developments in 

 animals is found in the trypanosomes. These protozoans evidently 

 arose from a form parasitizing insects, in which a mammal host 

 became included in the life cycle. By successive stages (Fig. 63), 

 the protozoans evolved into a form in which their entire life history 

 occurs in the mammalian host (Hoare, 1957). Some 20 species and 

 many other races and strains are known, ranging from the primitive 



Tronsmitted from horse to horse 

 mechanicolly in coitus 



Transmitted mechonicoily 

 by biting insects 

 -Development in soiivory qlonds 



-Development into midgut 



-No development post proboscis 



-Insect form only in onterior 

 stotion of digestive tract 



-Insect form in hind gut 

 or salivory glands 



Mommal added to life cycle; 

 Insect forms in hind gut 



Fig. 63. Probable phylogeny of certain trypanosomes, an example of extensive 

 evolutionary development within an apomyctic group. The large right branch 

 is the genus Trypanosoma. (Based on data in Hoare, 1957.) 



