48 I The Process of Evolution 



is homogametic. In many species the Y chromosome is lacking and 

 the male is then designated as XO. More complicated sex-chromo- 

 some mechanisms have evolved; these are discussed in Chap. 9. 

 For example, there are many sex chromosomes in some organisms. 

 It should be emphasized that the precise mechanism of sex deter- 

 mination varies from group to group, even though the chromosome 

 condition may appear the same. The evolution of sexuality as an 

 aspect of the storage of variability and its release through genetic 

 recombination is discussed in Chap. 8. 



Alterations of the Chromosomes 



In addition to the mutations discussed above (called gene or 

 point mutations), changes in the structure of the chromosomes take 

 place spontaneously and may be induced by the same agents that 

 cause gene mutation. Although these chromosomal alterations are 

 sometimes termed mutations, it is perhaps less confusing to restrict 

 the term mutation to gene changes. Chromosomal alterations also 

 are frequently referred to as aberrations or abnormalities. This is 

 because they are compared with an arbitrarily selected standard 

 chromosomal phenotype (usually the wild type); they should not 

 be taken to represent some unusual or deleterious phenomenon that 

 is inevitably disadvantageous to the organism. As with gene muta- 

 tions, chromosomal alterations usually have low or negative selective 

 value when they appear but may become stabilized in the population 

 or replace the standard type if their selective value increases as a 

 result of environmental change (if the nucleus and the genes are 

 included as part of the environment ) . 



Practically any accident that can be imagined as happening to the 

 chromosomes, during the course of cellular life and division, has 

 been found in laboratory organisms and organisms from the field. 

 Often such modification of behavior can be shown to have become 

 established as a regular feature of particular organisms. A simple 

 classification of chromosomal alterations would include deletions 

 (loss of a segment), duplications (repeat of a segment in a con- 

 tiguous or remote portion of the karyotype ) , inversions ( reversal of 

 a segment), and translocations (transfer of a portion of a chromo- 

 some to a nonhomologous chromosome, usually reciprocally). 

 Changes of chromosome number, often thought of as "chromosomal 

 mutation, " are discussed in Chap. 9. 



Deletions and Duplications. The role in evolution of loss or addi- 

 tion of chromosome material is very poorly understood. When homo- 

 zygous, deletions usually are lethal. They are a useful tool for map- 



