Larval habitats impose trait-dependent limits on the direction and rate of adult evolution in dragonflies

Abstract

Natural selection on juveniles is often invoked as a constraint on adult evolution, but it remains unclear when such restrictions will have their greatest impact. Selection on juveniles could, for example, mainly limit the evolution of adult traits that mostly develop prior to maturity. Alternatively, selection on juveniles might primarily constrain the evolution of adult traits that experience weak or context-dependent selection in the adult stage. Using a comparative study of dragonflies, I tested these hypotheses by examining how a species' larval habitat was related to the evolution of two adult traits that differ in development and exposure to selection: adult size and male ornamentation. Whereas adult size is fixed at metamorphosis and experiences consistent positive selection in the adult stage, ornaments develop throughout adulthood and provide context-dependent fitness benefits. My results show that species that develop in less stable larval habitats have smaller adult sizes and slower rates of adult size evolution. However, these risky larval habitats do not limit ornament expression or rates of ornament evolution. Selection on juveniles may therefore primarily affect the evolution of adult traits that mostly develop prior to maturity.

Keywords: carry-over effects; natural selection; odonate; ontogenetic conflict; sexual selection.

Figure 1.

Species that develop in lentic habitats have smaller adult sizes. (a) Phylogeny of focal species. Bars show adult size (mm) and are coloured by larval habitat (lentic = green, lotic = purple). Family abbreviations: Corduli = Corduliidae; Macrom = Macromiidae; Libell = Libellulidae; Petal = Petaluridae; Cordule = Cordulegastridae; Aeshn = Aeshnidae; Gomph = Gomphidae. (b) Estimated-marginal mean adult size (±s.e.) of lentic versus lotic species from phylogenetic generalized least-squares.

Figure 2.

Species that develop in lentic habitats are more likely to possess male wing ornamentation. (a) Phylogeny of focal species. Bars are coloured by larval habitat (lentic = green, lotic = purple/blue) and the species' possession of wing coloration (Y = dark, N = light). Family abbreviations: Corduli = Corduliidae; Macrom = Macromiidae; Libell = Libellulidae; Petal = Petaluridae; Cordule = Cordulegastridae; Aeshn = Aeshnidae; Gomph = Gomphidae. (b) Probability of male ornamentation (±s.e.) for lentic versus lotic species from phylogenetic logistic regression.