460 INTRODUCTION TO EVOLUTION 



heterozygous for the chromosome were actually more successful than were 

 homozygous "normal" flies. 



A comparable situation was found to exist in populations of Drosophila 

 pseudoobscura studied by Dobzhansky (1947, 1950). As background for 

 understanding of this study, it is necessary to recall that genes are ar- 

 ranged in a straight line down the length of a chromosome (a string of 

 beads is a useful analogy if not applied too literally). No one has yet seen 

 the genes, but there are ways of determining their locations relative to 

 each other. Within recent years the value for genetic studies of the rela- 

 tively huge chromosomes found in the cells of the salivary glands of 

 Drosophila larvae has been appreciated. These "giant chromosomes" are 

 characterized by an arrangement of cross-banding so varied in configura- 

 tion and arrangement of bands that each portion of each chromosome is 

 identifiable under the microscope. Furthermore, investigations, description 

 of which is outside the province of this book, have revealed that certain 

 bands are associated with the presence of certain genes. This is not to say 

 that the bands are the genes, but merely that the sequence in which the 

 bands occur along the chromosome may be taken as visual indication of 

 the sequence in which the genes occur in that chromosome. In the cells of 

 Drosophila there are four pairs of chromosomes; these are numbered for 

 convenience, one of the larger pairs being referred to as the third chro- 

 mosomes. In the investigation mentioned, Dobzhansky concentrated atten- 

 tion on the third chromosome of Drosophila pseudoobscura. He found that 

 in some individuals the bands, and hence the genes, on this chromosome 

 were arranged in one sequence, in other individuals in other sequences. 

 Inversions of longer or shorter sections of the chromosome were frequent. 

 For example, if we represent bands, or genes, by letters, we might have 

 a chromosome with the structure A BCDEFGHIJ. This might be 

 called the "standard" arrangement and be found in some individuals of a 

 race or species. Some other individuals might have the same genes in this 

 chromosome but have them arranged differently: the section C D E F 

 might be turned around, inverted, perhaps. Then the whole chromosome 

 would have the sequence: ABFEDCGHIJ. If both members of 

 this pair of chromosomes in an individual had the inverted arrangement, 

 the individual would be called an "inversion homozygote." If the individ- 

 ual possessed one uninverted or "standard" chromosome and one inverted 

 chromosome, that individual would be termed an "inversion heterozy- 

 gote." 



In the third chromosome of Drosophila pseudoobscura at least twenty- 

 one different gene sequences have been identified. Not all of these are 



