Natural selection on body size and proportions in house sparrows

Abstract

Variation in morphology across a species range is largely a function of climatic conditions. Evidence for this comes primarily from correlations of latitudinal trends of climate with those of body size or shape (the classic ecogeographic rules of Bergmann and Allen¹). Few studies have documented a temporal sequence of natural selection on size variables within a single population^2–7, and no study precisely documents selection on shape variables. For birds in temperate regions, winter climate can be a major source of mortality⁸, and is likely to be an important agent of selection on overall size^2–6. We reported previously⁶ that severe winter weather produced significant changes in the distribution of size classes of a population of house sparrows (Passer domesticus). In that study, post-winter males were on average significantly larger than pre-winter males and post-winter females averaged significantly smaller than pre-winter females. We report here further analyses indicating that sparrows also undergo selection for refinement of bodily proportions, consistent with predictions from Allen's ecogeographic rule. Such selection for increased core-to-limb proportions probably results in greater thermoregulatory efficiency. The changes in proportions accompany the changes in overall size⁶ and there are major inter-sexual differences in how such proportions are achieved.