BODY LENGTH 



THORACIC LENGTH 



WING LENGTH 



THORACIC WIDTH 



FIG. 19-4 Variations In body and wing measuremenh betw 

 two local populations of the gall wasp. Abscissas are in micr 



■es, in percentages of the population 

 (after Kinsey 1942). 



aberrations, the latter in the form of changes in the 

 number of chromosomes (haploidy, polyploidy) or 

 of arrangement of genes on the chromosomes (dele- 

 tion, duplication, translocation, inversion) (Dobzhan- 

 sky 1951 ). The rate at which any one gene mutates 

 varies greatly from one kind of gene to another, but 

 the average rate is of the order once in every 100,000 

 or 1,000,000 individuals. Between 0.4 and 10 per cent 

 of the individuals in each generation may possess mu- 

 tated genes. Natural populations may therefore be 

 well supplied with small nnitations of differing 

 potential values to the organism (Schmalhausen 

 1949). Mutations of different sorts apparently occur 

 haphazardly and are not influenced by environmental 

 conditions ; only accidentally do they give special ad- 

 vantages to an organism. Adaptations of a species 

 to a particular habitat or niche is effected through 

 natural selection of the favorable mutations out of the 

 many that occur. 



The size of the population and the rate at which 

 a particular gene mutates affect the odds that a mu- 

 tation of that gene will become established in a popu- 

 lation. In a stabilized population, two offspring must 

 survive and mature to replace the parents on their 

 death. If in one parent gene A mutates to the non- 

 lethal gene A' , the odds are I :1 that genotype A A' 

 will appear in one of their two offspring and that the 

 mutant gene will be transmitted to the following gen- 

 eration. If both offspring are heterozygous, the odds 

 for continuation of gene A' into the next generation 

 are increased to 3:1. On the other hand, if the parent 



carrying the mutant gene fails to reproduce or if all 

 the offspring die, the mutation is lost. The odds that 

 a single nmtation will persist through 127 generations 

 is estimated to be only 1 :67 (Fisher 1930). 



If the mutant gene A' is a dominant, the character 

 is immediately expressed in the phenotype ; if it is 

 recessive, the character will not appear in the pheno- 

 type until two heterozygous individuals mate to give 

 rise to the homozygous recessive A'A'. In a small 

 population, inbreeding between heterozygous indi- 

 viduals will quickly produce both homozygous domi- 

 nant and homozygous recessive genotypes, as well as 

 the heterozygous line, and provide a variety of pheno- 

 types upon which natural selection may work. In a 

 large population, mating between heterozygous indi- 

 viduals will be less frequent because these individuals 

 will constitute a lower proportion of the total popula- 

 tion. However, if the gene mutates repeatedly in 

 different individuals, the high mutation rate will 

 greatly increase the number of individuals carrying 

 the gene and increase the chances that the mutant 

 character will become expressed in the population. 



Hybridization 



The critical test of whether or not speciation 

 has occurred comes when a barrier between two geo- 

 graphically isolated populations breaks down, so that 

 the formerly isolated populations again come into 

 contact. If speciation is complete, they will not inter- 



Speciation 263 



