Knowledge of how spatiotemporal environmental variation impacts ecological and evolutionary processes and contributes to genetic and phenotypic diversity of natural populations is key to understanding and protecting biological diversity. In this thesis I used pygmy grasshoppers to study how environmental conditions, population dynamics, dispersal and admixture may influence genetic structure and diversity, and to evaluate how functionally important variation may affect the ability of populations to cope with novel and changing habitats.

Analyses of AFLP (Amplified Fragment Length Polymorphism) markers in Tetrix subulata individuals from 20 sampling locations in Sweden showed significant genetic structure and restricted gene flow among populations. Genetic diversity increased with population size and proportion of long-winged dispersive phenotypes on the island of Öland, but not on the mainland.

A contrasting environment comparative approach (CECA) applied to 20 T. undulata populations suggested that processes associated with environmental change differently influence functional and neutral diversity. Long-winged phenotypes were more common in disturbed than in stable habitats, indicative of recent establishment. Color morph diversity was higher in disturbed environments consistent with the notion that polymorphism promotes establishment success. Conversely, neutral diversity (AFLP) was lower in disturbed habitats, pointing to a stronger eroding effect of genetic drift in disturbed compared to stable habitats.

I compared genetic and morphological variation between sympatric populations of the two species. Populations of the generally dispersive T. subulata were genetically less differentiated compared with the more sedentary T. undulata, suggesting that the latter species has been less influenced by the homogenizing effects of gene flow. Non-parallel body size differences pointed to species-specific drivers of morphological change.

Finally, comparisons of reproductive output of T. subulata females that had been experimentally mated with males from the same or from a different population suggested that responses to interbreeding and genetic admixture can differ in direction and magnitude even between populations within a species, and thus influence whether dispersal translates into gene flow.

My thesis emphasizes the complexity of microevolution and illustrates how the effects of different ecological and evolutionary processes can vary according to disturbance regimes and geographic areas, and differ between closely related sympatric species.