The Source of Variability 77 



Drosophila pseiidoobscura and persimilis heterozygotes of various 

 inversions possessed greater vigor than their respective homozygotes 

 and further that the proportions of the various gene arrangements 

 varied in cychc fashion throughout the year. It appeared therefore 

 that the inversions represented gene rearrangements having shghtly 

 different adaptive quahties in different temperature bands, but 

 later Dobzhansky (1948Z?) found indications that the explanation 

 might involve gene change. 



In spite of the lack of experimentally proven adaptive effects, 

 chromosomal mutations have occurred in many species; they have 

 become homozygous in the population and have continued in their 

 new arrangement through long phylogenetic lines. This is the only 

 plausible explanation for the diversity of chromosome numbers and 

 arrangements found in many groups of organisms such as in 

 Drosophila ( Fig. 29 ) and the series of extreme chromosomal reduc- 

 tions found in the plant genus Crepis ( Babcock and Jenkins, 1943 ) . 



2. Gene Mutations 



In practically every population under careful genetic observation, 

 individuals appear which have a character not previously seen in 

 the population but a character which breeds true through sub- 

 sequent generations. Some of these characters are associated with 

 a change in chromosomal conditions or with the cytoplasm. The 

 others, constituting the great bulk of those observed, are presumed 

 to be caused by a change in the genie material itself and are called 

 gene mutations. 



So universal is this occurrence of gene mutations that mutability, 

 the ability to change, is considered an innate property of life. 

 Observed rates of mutation vary from group to group, from species 

 to species, and from gene to gene within a species (Beadle, 1955). 

 Beadle gives values for the incidence of mutations ranging from one 

 mutation in a thousand genes per life cycle to less than one in a 

 billion. Certain genes are said to increase the rate of mutation; 

 Schmalhausen (1949) contended that any mutations increase this 

 rate. 



LETHAL MUTATIONS 



A high proportion of all mutations are lethal. They produce a suf- 

 ficient disruption of normal physiology that they cause the death of 

 their carrier individuals before the latter reproduce. In nature these 

 mutations are weeded out of the population immediately. Examples 



