Natural Selection and Social Theory: Selected Papers of Robert L. Trivers

By Robert Trivers | Go to book overview

Natural Selection of Parental Ability to Vary
the Sex Ratio of Offspring
ROBERT L. TRIVERS AND DAN E. WILLARD

Abstract. Theory and data suggest that a male in good condition at the end of the period of parental investment is expected to outreproduce a sister in similar condition, while she is expected to outreproduce him if both are in poor condition. Accordingly, natural selection should favor parental ability to adjust the sex ratio of offspring produced according to parental ability to invest. Data from mammals support the model: As maternal condition declines, the adult female tends to produce a lower ratio of males to females.

Fisher (1) showed, and others (2) reformulated, that natural selection favors those parents who invest equally in both their sons and their daughters. When the parents invest the same in an average son as in an average daughter, natural selection favors a 50/50 sex ratio (ratio of males to females) at conception (3,4). (For simplicity, we assume here that parents are investing equally in average offspring of either sex. ) Individuals producing offspring in sex ratios that deviate from 50/50 are not selected against as long as these deviations exactly cancel out and result in a sex ratio at conception of 50/ 50 for the local breeding population. Such a situation is highly unstable, since random deviations from the 50/50 ratio in local populations rapidly favor those individuals producing their young in ratios of 50/50. We show here that under certain well-defined conditions, natural selection favors systematic deviations from a 50/50 sex ratio at conception, and that these deviations tend to cancel out in the local breeding population.

Imagine a population of animals (for instance, caribou) in which the condition of adult females varies from good to poor (as measured, for example, by weight). Assume that a female in good condition is better able to bear and nurse her calf than is a female in poor condition, so that at the end of the period of parental investment (PI), the healthiest, strongest, and heaviest calves will tend to be the offspring of the adult females who were in the best condition during the period of PI. Assume that there is some tendency for differences in the condition of calves at the end of the period of PI to be maintained into adulthood. Finally, assume that such adult differences in condition affect male reproductive success (RS) more strongly than they affect female RS. That is, assume that male caribou in good condition tend

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Natural Selection and Social Theory: Selected Papers of Robert L. Trivers
Table of contents

Table of contents

  • Title Page *
  • Preface vii
  • Contents *
  • Natural Selection and Social Theory *
  • 1 - Reciprocal Altruism 3
  • The Evolution of Reciprocal Altruism *
  • List of Literature *
  • Postscript *
  • 2 - Parental Investment and Reproductive Success 56
  • Parental Investment and Sexual Selection 65
  • Notes *
  • References 104
  • Postscript *
  • 3 - The Trivers-Willard Effect 111
  • Natural Selection of Parental Ability to Vary the Sex Ratio of Offspring 115
  • References and Notes *
  • Postscript *
  • 4 - Parent–offspring Conflict 123
  • Parent–offspring Conflict *
  • Notes *
  • References *
  • Postscript *
  • 5 - Haplodiploidy and the Social Insects 154
  • Haplodiploidy and the Evolution of the Social Insects - The Unusual Traits of the Social Insects Are Uniquely Explained by Hamilton's Kinship Theory *
  • References and Notes *
  • Postscript 206
  • 6 - Size and Reproductive Success in a Lizard 207
  • Sexual Selection and Resource-Accruing Abilities in Anolis Garmani *
  • Literature Cited *
  • Postscript *
  • 7 - Selecting Good Genes for Daughters 241
  • Asymmetry in the Evolution of Female Mating Preferences *
  • References *
  • Postscript *
  • 8 - Self-Deception in Service of Deceit 255
  • The Crash of Flight 90 - Doomed by Self-Deception? *
  • Postscript *
  • The Elements of a Scientific Theory of Self-Deception *
  • References *
  • Postscript 293
  • 9 - Genomic Imprinting 294
  • Genetic Conflicts in Genomic Imprinting *
  • Note 306
  • References *
  • Postscript 308
  • 10 - Fluctuating Asymmetry and the 2nd: 4th Digit Ratio in Children 309
  • The 2nd: 4th Digit Ratio and Asymmetry of Hand Performance in Jamaican Children 316
  • Notes *
  • References *
  • Postscript 328
  • References 329
  • Index 335
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