Selection in Natural Populations

By Jeffry B. Mitton | Go to book overview

3
Environmental Variability and Enzyme Polymorphism

It is clear to even the most casual observer that the environment is in a constant state of flux. It must be the case that fitness differences between genotypes are also in a constant state of flux.

J. H. Gillespie, The Causes of Molecular Evolution ( 1991)

The discovery of abundant protein genetic variation stimulated evolutionary biologists to take a more critical look at their models and to build more biologically realistic models. This reevaluation has included several largely independent goals. Two of the responses were at least partially prompted by the genetic load that would be generated if the variation were maintained by balancing selection. First, a quick response by three papers in the journal Genetics ( King 1967; Milkman 1967; Sved, Reed, and Bodmer 1967) questioned the traditional calculation of genetic load and devised models of fitness determination that generated little segregational load. Second, concern about an intolerable segregational load spurred the development and elaboration of a theory concerning the evolutionary dynamics of functionally equivalent, or neutral, allelic variation ( Crow and Kimura 1970; Kimura 1983; Kimura and Ohta 1971; Nei 1975). Third, emphasis shifted from single-locus models to multilocus models, and biological reality was increased to examine the effect of environmental heterogeneity on the distribution of genotypes.

This chapter begins by reviewing the models of environmental heterogeneity and concludes by examining the empirical studies of the influence of environmental heterogeneity on genetic variability.


Environmental Heterogeneity in Theory

Spatial heterogeneity

Levene ( 1953) first illustrated the importance of environmental heterogeneity for the maintenance of genetic variation. He modeled a randomly mating population distributed over several environmental patches that imposed different selection regimes on

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Selection in Natural Populations
Table of contents

Table of contents

  • Title Page iii
  • Preface vii
  • Contents ix
  • 1 - Natural Selection, Fitness Determination, and Molecular Variation 3
  • Summary 12
  • 2 - Classes of Abundant Genetic Variation 14
  • 3 - Environmental Variability and Enzyme Polymorphism 29
  • Summary 37
  • 4 - The Impact of a Single Gene 39
  • Summary 57
  • 5 - Patterns of Variation Among Loci 58
  • Summary 71
  • 6 - The Axis of Individual Heterozygosity: Theory 73
  • Summary 86
  • 7 - The Axis of Individual Heterozygosity: Empirical Data 87
  • Summary 126
  • 8 - Female Choice and Male Fitness 127
  • Summary 142
  • 9 - Patterns Among Species 144
  • Summary 156
  • 10 - The Sisyphean Cycle 157
  • Summary 166
  • 11 - Comments on Natural Selection 167
  • Appendices 175
  • Bibliography 199
  • Index 233
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