Increase in Number of Species 203 



whereas others will be represented by lower levels or stages of 

 division. Frequently, in the case of semi-distinct lines, it may be 

 difficult to ascertain whether two lines are becoming increasingly 

 distinct or are fusing. A puzzling and complex example of this is 

 found in the five subspecies described in Culicoides variipennis, a 

 North American biting midge of the family Ceratopogonidae (Wirth 

 and Jones, 1957). Each subspecies occurs in a somewhat homo- 

 geneous state in a relatively limited area; some may be separated 

 from each other on modal rather than absolute characters; and 

 populations of an intermediate nature occur between all of the 

 five subspecies. The relatively pure ranges of the five subspecies 

 (Fig. 88) are chiefly allopatric, but two, occidentalis and sonorensis, 

 overlap to a considerable extent. 



Of the five subspecies, typical C. variipennis is the most stable, 

 the best characterized, and probably the oldest; subspecies oc- 

 cidentalis is the next most distinctive; and the remaining three 

 subspecies sonorensis, albertensis, and australis form a closely knit 

 triad (Fig. 88). By combining these relationships with geographic 

 distribution, a simple sequence of events can be postulated. Prob- 

 ably the progenitor of the present species split first into central and 

 eastern populations. The latter eventually became typical vari- 

 ipennis. The central population subsequently divided into a far- 

 western population which evolved into subspecies occidentalis 

 and a mid-western population which was the parent of the triad 

 sonorensis-albertensis-australis. The subspecies australis has an 

 unusual amount of kaleidoscopic variation, reminiscent of the hybrid 

 silkworm Platysamia kasloensis, and suggesting that subspecies 

 australis is a hybrid entity. This can be explained by assuming that 

 the inland ancestral population of the triad divided into two enti- 

 ties, a southwestern and a Great Basin population (sonorensis) and 

 a Great Plains population (albertensis), and that finally, possibly 

 late in the Wisconsin glacial period, the ranges of subspecies vari- 

 ipennis, albertensis, and sonorensis spread into a common south- 

 central area. Presumably sexual isolation between these three types 

 is as yet somewhat undeveloped, resulting locally in free hybridiza- 

 tion and the formation of a highly variable hybrid swarm, the sub- 

 species australis. The resultant phylogeny is shown in Fig. 88. 



Marked ecological differences exist between the subspecies. C. 

 V. albertensis, australis, and usually occidentalis inhabit saline situa- 

 tions, whereas variipennis and sonorensis breed in heavily polluted 

 but not unusually saline breeding areas. As suggested by Wirth 

 and Jones, the whole species may represent a somewhat freely 



