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Duchesne, Megan

University of British Columbia

Novel interactions among native and colonist species are now common due to range shifts in response to climate and land use change, but few studies of native populations are sufficiently detailed or long to document reliably their evolutionary and demographic responses to colonization by controphic competitors. I emulated a classic study of competition, rapid evolution, and coexistence in Galapagos Finches (Geospiza) to test if the colonization of XOXDEL (Mandarte I), British Columbia, Canada, by Fox Sparrows (Passerella iliaca unalaskensis), a dominant controphic competitor to the native Song Sparrow (Melospiza melodia) resident there, led to directional selection on bill size or shape and subsequent convergence on or divergence from bill morphology of Fox Sparrows over 49 years (1975-2023). To do so, I used 3,991 captures of 1,554 adult and yearling Song Sparrows to test for natural selection and morphological evolution. Survival increased with bill size in adult and yearling males and yearling females on average, effectively truncating distributions in adult male and female breeders that hatched locally. However, despite strong directional selection and positive relationships between annual selection differentials and change in the mean bill size and shape of breeding adults across years, I did not observe divergence or convergence in the bill sizes or shapes of Song and Fox Sparrows. Although I also predicted that the intensity of natural selection on bill morphology would increase with the density of intra- and/or interspecific competitors, those predictions were not supported. I did not find support for the hypothesis that large bills enhance survival by reducing heat stress. While ecological conditions were situationally similar to those described by Grant & Grant (2006), patterns of natural selection and morphological change remained unpredictable despite substantial variation in individual phenotype, survival, and competitor density. My results advance understanding by providing clear examples of the influence of natural selection on bill morphology in a small, insular population of Song Sparrows that was colonized by a superior competitor for seeds in 1975 but declined to extinction over the subsequent half-century.

Text

2025-04-25

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/90815

Forestry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/