Micro- and macroevolutionary aspects of body size diversification and thermal adaptation in insects

Abstract: Body size and body temperature are the two most important traits in biology. In this thesis I show how these twovariables have shaped the evolution of insects over the last 300 million years. These variables are so important infact that three rules have been developed to summarize their effects: (1) bigger is better, (2) hotter makes yousmaller, and (3) hotter is better.Using primarily dragonflies and damselflies, I explore the implications of these rules in the evolution of insects.Bigger is better (rule 1) states that larger individuals tend to have higher fitness. In insects larger adult body size isexpect to produce higher fitness in terms of higher fecundity in females and higher mating success in males, whilepotentially increasing longevity in both males and females. I show that there are limits to how large one can get, andstabilizing selection body is expected to be the norm in most populations, mediated through conflicting selectionbetween large adults and costs of increased development time.Hotter makes you smaller (rule 2), also known as the temperature-size rule in insects, states that within a populationinsects that develop in warmer regions will be smaller. The hotter makes you smaller rule is typically observed alonga latitudinal gradient within the range of a species. This within species trend suggests strong associations betweenan organism size and its developmental environment, and suggests a possible resolution to of the paradox imposedby the bigger is better rule. While the temperature-size rule is well-established in natural and laboratory populations,I show that other factors such as predation and dispersal are likely determining latitudinal size patterns betweenspecies.The final rule (rule 3), hotter is better, is the least well-established “rule” of the three rules. It states that warmer bodytemperatures will tend to lead higher performance in adult insects. Since dragonflies and damselflies areectotherms, they are strongly dependent on ambient temperature to achieve optimal body temperatures, I show thathotter is not always better, and individuals with higher heating rates are favoured only in extreme temperatureconditions, and otherwise thermal inertia is preferred, since the mean environment will match an individual’s optimalbody temperature.