The Source of Variability 87 



acter change observed in evolutionary studies. Two lines of evi- 

 dence show the fallacy of this idea. 



In the first place, every natural population adequately tested 

 contains an abundance of mutant alleles of the same types as those 

 found in the laboratory. Regarding Drosophila, Spencer (1947) 

 states that wild or semi-wild populations carry many more than the 

 1,000 or more mutants reported from experiments and that certain 

 kinds of mutants can be obtained more easily from wild populations 

 than by radiation or other laboratory methods. This coincidence of 

 wild and artificially induced mutation indicates that all the different 

 alleles occurring in nature arose originally as mutant types such as 

 are studied in the laboratory. Special techniques of detection have 

 shown also that the lethal and sublethal alleles also abound in 

 natural populations (Spencer, 1947). 



Numerous natural alleles have been observed. In humans, a mu- 

 tant for a six-fingered hand occurs with some frequency. Mosquitoes 

 show many allelic components similar to those found in other or- 

 ganisms (Lavan, 1957). 



Many natural physiological traits are allelic in nature, suggesting 

 that they arose as mutations in nature. Harvey (1954) found that 

 in the spruce budworm Choristoneura fumiferana about 20 per cent 

 of some populations did not diapause, and that by mating selection 

 this fraction could be increased to 80 per cent. In the larch sawfly 

 Pristiphora ericJisoni one race possesses an allele for encapsulating 

 and killing the larvae of certain internal parasites ( Muldrew, 1953 ) . 

 Examples in the plants include mutants of wheat rust which will 

 attack previously resistant races of wheat, and mutants of the mold 

 Penicillitim which produce different amounts of the antibiotic 

 penicillin. 



The second major point concerning these mutations is that many 

 of them do represent the kind of differences found between natural 

 taxa. 



In the plant Primula sinensis an assortment of laboratory mutant 

 types of leaves, bracts, calyx, and corolla display differences in 

 shape much like those found between distinct plant species (Fig. 

 35 ) . In the fly Drosophila, laboratory mutant types include the ad- 

 dition or loss of setae, juxtaposition of wing veins, and many color 

 characters on almost all parts of the body (Patterson and Stone, 

 1952), and these are the type of character used to differentiate 

 many species of the genus. Stebbins (1950) lists many more ex- 

 amples for plants. 



The short- winged mosquito mutants observed by Lavan (1957) 



