Selection on mothers and offspring: whose phenotype is it and does it matter?

Area of Science:

• Evolutionary biology
• Quantitative genetics
• Animal ecology

Background:

• Reproductive and early life-history traits are key to offspring and maternal fitness.
• Evolution of these traits depends on selection acting on both offspring and maternal components of fitness.
• Antagonistic selection can occur when optimal phenotypic values for offspring and maternal fitness differ.

Purpose of the Study:

• To investigate selection regimes on correlated reproductive traits (birth weight, birth date, litter size) in a free-living Soay sheep population.
• To test the hypothesis that antagonistic selection, acting at offspring and maternal levels or on correlated traits, imposes evolutionary constraints.
• To understand how assigning traits to offspring or maternal phenotype influences selection.

Main Methods:

• Utilized long-term study data from a free-living Soay sheep (Ovis aries) population in St. Kilda, Scotland.
• Analyzed selection on birth weight, birth date, and litter size using variance in short-term and lifetime fitness measures.
• Examined selection at both offspring and maternal levels to identify antagonistic pressures.

Main Results:

• All three traits (birth weight, birth date, litter size) were under selection impacting fitness.
• Lifetime fitness analysis showed positive directional selection for birth weight and birth date.
• Evidence of stabilizing selection on birth weight and birth date at the maternal level, with lower optima than for offspring.
• Antagonistic selection on litter size: singletons favored by offspring, larger litters by mothers.
• Reduced fitness of twins linked to lower birth weight, indicating trade-offs.

Conclusions:

• Selection regimes on reproductive traits are complex and depend on whether traits are viewed as offspring or maternal.
• Antagonistic selection between offspring and maternal fitness optima creates evolutionary constraints.
• Trade-offs between litter size and birth weight, with differing optimal resolutions for mothers and offspring, drive antagonistic selection.
• Understanding the dual roles of traits in offspring and maternal fitness is crucial for evolutionary studies.