Evolution in changing environments revealed by fire melanism in pygmy grasshopper

Sammanfattning: The effect of spatio-temporal environmental variation on within population genetic diversity is interesting from both theoretical- and applied perspectives such as conservation biology and wildlife management. Theory predicts an interactive process whereby environmental variation may maintain genetic variation, and genetic and phenotypic polymorphism may enhance the ability of populations to cope with stress imposed by changing environments. Pygmy grasshoppers (Tetrix subulata) constitute a useful model system for exploring effects of environmental change on genetic variation in natural populations because they are highly polymorphic for colour patterns and sometimes thrive in fire ravaged environments. In my thesis I investigate if the relative proportion of black individuals differ between populations in recently burnt compared to non-burnt areas and how population composition changes over time. I perform experiments to test if colour patterns in these insects are strictly determined by genotype or influenced also by developmental plasticity in response to the environment experienced during growth. I also investigate whether average survival among individuals in diverse groups is higher than survival rates in less variable groups, as would be expected if competition is less intense in diverse groups. My results show that the frequency of black and very dark individuals on average is higher in recently fire ravaged areas compared with non-burnt areas. The proportion of melanistic individuals is highest the first year after a fire. After the initial increase, the proportion of black individuals declines between years and the distribution among colour morphs becomes more even. Experiments show that colour patterns in pygmy grasshoppers are not influenced by environmental cues such as burnt material and high population densities that are common in the post fire environment, indicating that colour is mainly determined by genes in these insects. Finally, I show that reduced competition among alternative colour morphs may influence the change towards a more even distribution among colour morphs a few years after a fire. Taken together, my results show that evolution by natural selection can be a very quick process and support the idea that polymorphic populations are able to endure environmental changes and adapt quickly to novel environmental conditions caused by random events such as forest fires.