Opposing selection and environmental variation modify optimal timing of breeding

Area of Science:

• Evolutionary biology
• Life history theory
• Quantitative genetics

Background:

• Phenotypic traits influencing both fecundity and viability can lead to opposing selection, a paradox in evolutionary studies.
• Life history theory posits tradeoffs between fitness components, yet empirical investigation in selection studies is limited.
• Breeding timing is a heritable trait often under directional selection without proportional population response, suggesting underlying complexities.

Purpose of the Study:

• To investigate opposing viability and fecundity selection on breeding timing in a wild population.
• To determine how environmental factors and individual experience modify selection pressures.
• To understand the implications of life history tradeoffs for evolutionary responses and optimal breeding strategies.

Main Methods:

• A 22-year longitudinal study of a tropical parrot population.
• Analysis of heritable phenotypic traits, fecundity (offspring production), and viability (survival).
• Statistical modeling to account for sampling error and assess selection gradients.

Main Results:

• Opposing viability (positive) and fecundity (negative) selection on breeding timing was consistently observed.
• Selection magnitude fluctuated annually, influenced primarily by environmental conditions (rainfall, breeding density) and secondarily by breeding experience.
• Late-born juveniles exhibited greater survival, and later breeding increased fitness under opposing selection.

Conclusions:

• Life history tradeoffs significantly influence selection on phenotypic traits like breeding timing.
• Unifying selection and life history theory is crucial for a comprehensive understanding of evolution.
• Monitoring both survival and reproduction is essential for predicting phenological responses to environmental change.