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Evolutionary dynamics of the Trivers-Willard effect: A nonparametric approach.

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  • 1University of Bamberg Bamberg Germany.

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Parents adjust offspring sex ratios based on condition, influencing reproductive values. This study reveals Trivers-Willard effects can evolve to be much larger than predicted, with complex dynamics.

Keywords:
Trivers–Willard hypothesiscontinuous strategiesevolutionary dynamicsevolutionary invasion analysisintegral projection modelstwo‐sex models

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Area of Science:

  • Evolutionary biology
  • Behavioral ecology
  • Population dynamics

Background:

  • The Trivers-Willard hypothesis (TWH) predicts parental condition influences offspring sex ratio bias.
  • Existing models often overlook how offspring sex ratio adjustments affect population-level reproductive values.

Purpose of the Study:

  • To model the full evolutionary dynamics of the Trivers-Willard effects (TWE).
  • To investigate how dynamic feedback between sex ratio and reproductive value influences evolutionary trajectories.
  • To analyze the evolution of continuous strategies in structured populations.

Main Methods:

  • Utilized a two-sex integral projection model for Columbian ground squirrels (Urocitellus columbianus).
  • Treated offspring sex ratio as a continuous function of maternal condition.
  • Simulated evolutionary change via successive invasion of mutant strategies until an evolutionarily stable strategy (ESS) was reached.

Main Results:

  • The magnitude of evolving TWE can be significantly greater than previously predicted.
  • Evolutionary dynamics exhibit substantial nonlinearities, influenced by starting conditions.
  • Evolutionary change is fastest from extreme initial sex ratios and weaker for high-condition parents.

Conclusions:

  • Dynamic feedback is crucial for understanding natural selection and TWE evolution.
  • The study provides a novel methodological framework for analyzing continuous strategies in structured populations.
  • Evolving TWE does not necessarily maximize population fitness or minimize sex ratio deviation.