Large body size in an island-dwelling bird: a microevolutionary analysis

Area of Science:

• Evolutionary biology
• Island biogeography
• Quantitative genetics

Background:

• Island populations of passerine birds frequently evolve larger body sizes than their continental relatives.
• The Capricorn silvereye (Zosterops lateralis chlorocephalus) has shown significant body size increase since island colonization approximately 4000 years ago.

Purpose of the Study:

• To investigate the genetic basis and selection pressures driving the observed increase in body size in the Capricorn silvereye.
• To determine if the genetic variance-covariance (G) matrix and predicted evolutionary responses align with the island passerine trend of larger body size.

Main Methods:

• Estimation of the genetic variance-covariance (G) matrix using full-sib and animal model analyses.
• Calculation of selection gradients for six morphological traits in three consecutive cohorts of nestlings under field conditions.

Main Results:

• Significant genetic variance was detected for all measured morphological traits.
• Directional selection was observed for wing and tail lengths in one year, and quadratic selection on culmen depth in another year.
• Despite negative selection gradients for some traits, the predicted evolutionary response for all cohorts was consistently positive, indicating a trend towards larger body size.

Conclusions:

• The G matrix and predicted evolutionary responses in the Capricorn silvereye support the hypothesis of evolving towards larger body size.
• These findings are consistent with the general pattern of island passerine evolution observed globally.