The ecology of adaptive condition-dependent polyphenism

Organismal development is generally sensitive to environmental cues and perturbations. One common mode of such plasticity is adaptive condition-dependence, where development responds adaptively to cues from the developing phenotype itself, mitigating current problems and anticipating future challenges. Such adaptive condition-dependent plasticity can be a subtle buffering mechanism that simply canalizes development in the face of environmental variation. However, when individuals differing in condition experience distinct sets of ecological affordances, adaptive condition-dependent plasticity can result in spectacular phenotypic variation. A prominent example of this is heightened condition-dependent variation in arthropod ornaments and armaments, in which condition-dependence regularly takes the form of polyphenism with discrete ‘minor’ and ‘major’ morphs. In this dissertation, I discuss the evolution of condition-dependent polyphenism in arthropod secondary sexual traits. I first review the evidence for ecological mechanisms causing selection for condition-dependent polyphenism in arthropods. I then use the male-polyphenic acarid mite Rhizoglyphus robini as a case study to investigate how the ecological environment affords different opportunities to poorly-growing and well-growing males, and how polyphenic development of secondary sexual traits (enlarged and modified legs used as weaponry in intrasexual fights, generally absent in poorly growing males) responds to body size and growth rate. I present evidence that relatively small males are less likely to win fights and benefit less from weaponry than larger males; polyphenism thus prepares for an alternative mating tactic. Additionally, slowly growing males accelerate their life history pace by not developing weaponry; polyphenism thus protects maturation age against variation in somatic growth. Understanding how development adaptively maintains the capacity to either canalize or amplify phenotypic variation through adaptive condition-dependence is key to understanding evolutionary shifts in form and function, as phenotypic variation is the raw material for evolution by natural selection.