Density-dependent environments can select for extremes of body size

Abstract

Body size variation is an enigma. We do not understand why species achieve the sizes they do, and this means we also do not understand the circumstances under which gigantism or dwarfism is selected. We develop size-structured integral projection models to explore evolution of body size and life history speed. We make few assumptions and keep models simple: all functions remain constant across models except for the one that describes development of body size with age; size at sexual maturity is constant at 80% of asymptotic size across life histories; and density-dependence impacts only reproduction or juvenile survival. Carrying capacity is fitness in the models we develop and we are consequently interested in how it changes with size at sexual maturity, and how this association varies with development rate. The simple models generate complex dynamics while providing insight into the circumstances when extremes of body size evolve. We identify different areas of parameter space where gigantism and dwarfism evolve. The direction of selection depends upon emergent trade-offs among the proportion of each cohort that survives to sexual maturity, life expectancy at sexual maturity, and the per-capita reproductive rate. The specific trade-offs that emerge depend upon where density-dependence operates in the life history. Empirical application of the approach we develop has potential to help explain the enigma of body size variation across the tree of life.