Contents xiii 



Selection 102 



Fitness or Adaptive Value 104 



Types of Selection 104 



Homozygous Recessives Completely Unsuccessful 105 



Homozygous Recessives Relatively Unsuccessful 108 



Homozygotes Inferior to Heterozvgotes 109 



Balanced Polymorphism and the Retention of Variability 110 



Genetic Load 112 



Heterozygotes Inferior to Homozygotes 114 

 Migration and Population Structure 114 

 Joint Pressures 116 

 Adaptation and Gene Combinations 120 

 Summary 122 

 References 123 



Chapter 7 Changes in Populations 124 



Examples from Nature 125 



Differentia] Mortality in Sparrows 125 



Industrial Melanism 125 



Microevolution in British Lepidoptera 131 



Polymorphic Land Snails 133 



Island Water Snakes 137 



Chromosomal Polymorphism in Drosophih 139 



Examples from Man 145 



Pasture Plants 146 



Mimicry of Flax 147 



Disrupti\e Selection in Mimetic Butterflies 147 



Resistance to Antibiotics and Insecticides 148 

 Laboratory Populations 150 

 Genetic Homeostasis 153 

 Genetic Assimilation 154 

 Adjustment to the Environment 157 

 Summary 158 

 References 158 



Chapter 8 I Genetic Systems I 160 



Genetic Systems in Microorganisms 161 

 Transformation 161 

 Recombination in Viruses 162 

 Transduction 163 



