152 Species and Species Change 



by the cheetah, displays the reverse of other hues, bemg a running 

 plains animal with many dog-like habits (Wright, 1960). 



The caddisflies Sortosa and Chimarra offer an example of tem- 

 perature reversal. The progenitor of Sortosa was undoubtedly a 

 cool-adapted species, as the many living members of Sortosa still 

 are. From this form evolved the warm-adapted genus Chimarra. 

 In its early evolution Chimarra became a widespread and abundant 

 tropical caddisfly, but at a later time one of its branches evolved 

 into a group of cool-adapted forms now living in company with 

 Sortosa. The phylogenetic line of each of these later species traces 

 back through a tropical ancestor to the original cool-adapted parent. 



THE ROLE OF BIOTIC FACTORS IN ADAPTIVE CHANGE 



In many of the examples of natural selection and evolutionary 

 change mentioned in this and the preceding chapter, competition 

 is mentioned prominently. It may also have been important in some 

 examples such as Chimarra. Competition, however, is not the only 

 biotic factor which will change the pattern of selection pressures 

 operating on a species. In parasitism, predatism, symbiosis, com- 

 mensalism, and other circumstances, ecological interaction between 

 different species may affect profoundly the future evolution of one 

 or all of the species involved. Biotic factors therefore loom as tre- 

 mendously important in the processes of adaptive change. It is 

 also evident that the more species there are, the greater will be 

 the complexities of this relationship. Thus attention is drawn force- 

 fully to the subject of the increase in number of phylogenetic lines, 

 or species. 



