142 Species and Species Change 



ostasis. On the basis of present evidence and inference these changes 

 fall into two categories: new unusual genetic constituents, and 

 reduction in complexity of the gene pool. 



NEW UNUSUAL GENETIC CONSTITUENTS 



The type of mutation most likely to be perpetuated would have 

 only a slight effect on the physiology of the genetic system but a 

 marked positive selective effect on the phenotype. Two examples 

 illustrate this possibility. Discussing the rapid early evolution of 

 the dipnoian fishes, Westoll (1949) suggested that the early forms 

 may have had some simple genetic mutations which acted on early 

 developmental processes and produced relatively large or varied 

 phenotypic effects. He pointed out that if these were selectively 

 advantageous, they could have changed the spectrum and action 

 of further selection to a high degree. In the caddisfly order Tri- 

 choptera, the evolution of the entire case-making suborder appears 

 to fit Westoll's premises perfectly. The adults of the entire order 

 are essentially terrestrial-aerial; the larvae are aquatic and in one 

 line are either naked or make portable cases. The mature naked 

 larvae build an outer cocoon, shaped like a tortoise shell, from 

 small stones and cemented to a support. Within it they spin an 

 inner cocoon in which they pupate. From this type arose a form 

 which builds the outer stOne cocoon when very young and carries 

 it around as a case; when full grown the larva cements its outer 

 cocoon or case to a stone, spins the inner cocoon within it, then 

 pupates. From a form making this crude type of case evolved a 

 great number of lines making cases progressively better constructed 

 from many utihtarian standpoints (Fig. 59) (Ross, 1956). 



Starting from a naked-larva parent, the entire case-making line 

 could have begun with a single mutant individual in which the 

 instinct for building the outer cocoon had been switched from the 

 mature larva to the newly hatched larva. For survival such a 

 mutant would need to be favored by a strong positive selection 

 pressure, and this circumstance was presumably protection from 

 predation afforded by the stone case. Such protection also allows 

 these larvae to crop diatoms, their chief food, not only from their 

 protected hatching areas but also from exposed surfaces in the 

 stream. The added grazing area would result in increased popula- 

 tion size with its impetus to a greater rate of adaptive genetic 

 change. 



The postulate in both the fish and caddisfly examples is that 

 mutations occurred having (1) slight genetic effect hence a mini- 



