On the role of sex differences for evolution in heterogeneous and changing fitness landscapes: insights from pygmy grasshoppers

Abstract

Much research has been devoted to study evolution of local adaptations by natural selection, and to explore the roles of neutral processes and developmental plasticity for patterns of diversity among individuals, populations and species. Some aspects, such as evolution of adaptive variation in phenotypic traits in stable environments, and the role of plasticity in predictable changing environments, are well understood. Other aspects, such as the role of sex differences for evolution in spatially heterogeneous and temporally changing environments and dynamic fitness landscapes, remain elusive. An increased understanding of evolution requires that sex differences in development, physiology, morphology, life-history and behaviours are more broadly considered. Studies of selection should take into consideration that the relationships linking phenotypes to fitness may vary not only according to environmental conditions but also differ between males and females. Such opposing selection, sex-by-environment interaction effects of selection and sex-specific developmental plasticity can have consequences for population differentiation, local adaptations and for the dynamics of polymorphisms. Integrating sex differences in analytical frameworks and population comparisons can therefore illuminate neglected evolutionary drivers and reconcile unexpected patterns. Here, I illustrate these issues using empirical examples from over 20 years of research on colour polymorphic Tetrix subulata and Tetrix undulata pygmy grasshoppers, and summarize findings from observational field studies, manipulation experiments, common garden breeding experiments and population genetics studies.This article is part of the theme issue ‘Linking local adaptation with the evolution of sex differences’.

Contrasting patterns of neutral and functional genetic diversity in stable and disturbed environments

“…By virtue of their dispersal polymorphism, color polymorphism, life history, and transient, habitat‐tracking, meta‐population like dynamics, the well‐studied pygmy grasshoppers lend themselves admirably for investigating drivers of population‐level genetic diversity (Forsman, ; Tinnert & Forsman, ). Overall, our present results illustrate how the genetic structure and diversity of natural populations are influenced by a combination of ecological and evolutionary processes, the relative importance of which differ for neutral and functional genetic diversity and vary according to ecological conditions, such as environmental disturbance regime.…”

“…Habitats in stable environments included stream shorelines, pastures, meadows, and agricultural land, all of which are influenced by small‐scale substrate surface modifications through trampling by farm animals, wave action, grazing by cattle and wild birds (e.g., geese and ducks), and agricultural activities. The modifications of soil surface and ground vegetation that result from these activities favor pygmy grasshopper behaviors, such as feeding and egg‐laying (Forsman, ). In contrast, sampling localities in disturbed environments represented habitats affected by more drastic and larger scale environmental makeovers associated with previous clear cuttings, natural forest fires, or managed fires.…”

“…The conversion of forests, which are largely unsuitable as pygmy grasshopper habitats, through fires and cutting to open land with patches of bare soil and relatively few enemies, provides new habitats available for colonization, predominantly by long‐winged flight‐capable phenotypes. Overall, some species of pygmy grasshoppers seem well adapted to using a tracking strategy suitable for spatiotemporally changing environments and can thrive under these conditions (Forsman, ).…”

“…Females survive at most one reproductive season, produce multiple pods of egg (<35 eggs/clutch), and nymphs develop through five (males) or six (females) instars before eclosing. Because of its high reproductive capacity, T. undulata may rapidly become very numerous when conditions are favorable (Forsman, ).…”

“…There is experimental evidence from pygmy grasshoppers that higher levels of color polymorphism protect individuals and populations against predation (Karpestam, Merilaita, & Forsman, ), reduce adverse effects of intraspecific competition (Caesar, Karlsson, & Forsman, ), and promote establishment success of founder groups (Forsman et al., ; Wennersten, Johansson, Karpestam, & Forsman, ). There is ample evidence that pygmy grasshopper color morphs are genetically influenced and not affected to any important degree by developmental plasticity (see Forsman, Karlsson et al., ; Forsman, , references therein and Methods below).…”