As has been seen in the consideration of evolutionary change in the 

 quantitatively measured characteristics of organisms, recombination 

 is a factor more significant than mutation as a source of variability. 

 Since the genotype of an organism evolves under conditions de- 

 termined, among other things, by the constitution and organization 

 of its chromosomes, the investigation of the chromosomal mechanism 

 is particularly important in evolutionary studies. The basis of \aria- 

 tion lies in the genetic code, but changes in the structure and num- 

 ber of the chromosomes bearing the code may directly or indirectly 

 affect the amount of recombination of the code. 



Genes are defined here as those regions of the chromosome be- 

 tween the closest points of crossing-over. They are therefore the 

 smallest units of recombination in higher organisms. Larger units of 

 recombination may involve particular portions of the chromosome 

 in which crossing-over or its effects are restricted; these may be 

 called supergenes. There are whole chromosomes, e.g., the sex 

 chromosomes, which behave as recombinational units. The entire 

 nucleus is the unit of transmission and recombination in organisms 

 that reproduce asexually (some kinds of apomictic organisms). 



The cytological mechanisms that determine, in part, the amount 

 of recombination in a population are an important aspect of the 

 genetic system of the organism. Often they are visible in the pheno- 

 tvpic appearance of the chromosomes, the karyotype, flowever, 

 structural and genie changes may occur without any obvious changes 

 in the karyotype. The cytological mechanisms about to be discussed 

 usually are referred to as aberrations or imitations. Since they ap- 

 pear to occur with measurable frequency in most organisms and 

 since they become characteristic of entire populations or taxonomic 

 groupings of animals and plants, they are best thought of as aspects 

 of the cytogenetic repertoire of organisms. As with any other char- 

 acteristic, they arise spontaneously and are maintained by selection 

 as long as they confer an advantage. 



MEIOTIC DRIVE 



In a variety of organisms, all alleles of a locus do not have an equal 

 chance of inclusion in the gametes. This phenomenon has been 

 called meiotic drive, since one allele is favored or driven. It is part 

 of the genetic system, as it reduces the amount of recombination. 

 Cytogenetic mechanisms that affect the distribution of alleles have 

 been studied in some detail in Drosophila melanogaster. It is obvi- | 175 



