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UBC Theses and Dissertations

Patterns and drivers of selection on laying date in song sparrows (Melospiza melodia) Essak, Martha


Natural selection in the wild has been extensively researched, but few studies have identified the abiotic and biotic drivers of selection or quantified their influence. I characterized and quantified annual selection on laying date in the song sparrow (Melospiza melodia) population of Mandarte Island, British Columbia over 35 years. Selection was approximately linear for annual reproductive success (ARS) and over-winter survival based on cubic splines visualization of fitness surfaces. Fecundity selection (recruited offspring) was roughly 10 times stronger (-0.271 ± 0.031, mean ± SE) than viability selection (-0.028 ± 0.043, mean ± SE). Since opposing selection could constrain evolution, I used multiple measures of fitness, including fecundity measured at sequential offspring life stages, and female over-winter survival, to test if selection was complementary or opposing. Selection favored early breeding through all fitness measures and was therefore complementary. The strength of fecundity selection on recruits was 1.43 times stronger than selection on nestlings, indicating that the effects of early laying were additive and accumulated over time. Despite strong selection and moderate heritability (h² = 0.16), there has been no phenotypic advance of laying date in this population from 1975 to 2010. I also investigated several potential drivers of selection including density, spring temperature, precipitation and the intensity of brood parasitism by brown-headed cowbirds (Molothrus ater), and determined their relative influence on fecundity and viability selection. I found that density was the most influential driver of fecundity selection, and that selection favoring early breeding increased in magnitude at high population density and high intensities of brood parasitism. Because population density and the age and inbreeding structure of this population are correlated, I also included each of these variables as a covariate in calculation of selection differentials, and found weaker selection for some fitness measures. A climate index representing precipitation during the pre-laying period was the most influential driver of over-winter viability selection, with early breeding favored in winters preceded by warm, dry springs, and late breeding favored in winters preceded by cool, wet springs.

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