Labile characters, like behaviors, are phenotypes that are expressed repeatedly in the life of an individual. These types of characters allow individuals to adjust their phenotype to various levels of environmental variation, and therefore play a key role in the evolutionary process. Labile phenotypes are distinct because of their multi-level nature; individuals can differ in their average phenotypic expression (causing among-individual variation), but they can also vary their phenotype in each

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... expression (causing within-individual variation). In order to understand the role of labile characters in the evolutionary process it is necessary to acknowledge that variation at each level is caused by different processes. Variation at the among-individual level is caused by genetic or environmental differences having a permanent effect on an individual's phenotype, whereas variation at the within-individual level is caused by an individual's adjustment of its phenotype to a changing environment. The implications of these multi layered effects in the expression of labile characters have been acknowledged by different fields of evolutionary ecology, but major areas of evolutionary research do not fully incorporated this idea. The general aim of my thesis was to fully integrate this multi-level nature in the study of the adaptive causes and evolutionary consequences of variation in labile characters. My thesis is composed of five chapters: the first three are conceptual and methodological works aimed at integrating the multi-level nature of labile characters into already existing evolutionary frameworks. The last two chapters describe, as a worked example, how the different levels of variation and covariation between (labile) fertilization related traits affect the evolution of the alternative reproductive strategies in a wild passerine bird (the great tit). The first chapter is a conceptual work focusing on how to define and statistically characterize behavioral characters. We argue that behavioral characters can be studi [...]