Abstract
The need for evolutionary studies on quantitative traits that integrate genetics, development and fitness consequences is increasing. Due to the complexity, coherence and variability of behavioural traits, evolutionary biologists are therefore more and more attracted to the study of behaviour. The use of the model system of consistent individual differences
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in personality traits in the great tit provides a good foundation to do controlled experiments on the mechanisms underlying the variation in complex behavioural traits, and to make the step to functionality and evolution. The study presented in this thesis is part of a NWO program on the heritability, ontogeny and fitness consequences of personalities. The genetic background and the structure of the genetic mechanism that underlies the inheritance of these personality traits were investigated in this study.
In our study on personality traits in the great tit we have been able to breed animals experimentally and to artificially select for our traits of interest. We show with two independent artificial selection experiments that a significant part of the phenotypic variation in early explorative behaviour (chapter 2) and in risk-taking behaviour (chapter 4) can be ascribed to variation in the genetic make-up of these traits. We found a difference in the realized heritability between the two traits (54% for early exploratory behaviour and 19% for risk-taking). Since risk-taking behaviour is measured in a later stadium of a bird’s life this difference could therefore be seen as an indication for the existence of learning effects in personality traits.
With different methods we found different heritability estimates in our study. A possible cause for this variation in heritability estimates would be the existence of nonadditive or indirect genetic effects (IGEs) in the inheritance of these traits. The analyses of line crosses enabled us to separate the components of variation. The analysis (chapter 3) shows that besides a considerable amount of additive genetic variation also genetic dominance plays an important role in the structure of inheritance. Since we did not detect sex-dependent expression in this analysis, we can assume that differences between sexes that have been found are due to differences in selection pressures or interactions with the social environment (chapter 5), rather then a difference in expression of the same genes. Our analyses also showed that the part of the phenotypic variation that could be explained by heritable additive maternal effects was relatively low (7 %).
In great tits, explorative behaviour showed to be phenotypically correlated with many other traits within the same context. Moreover, we measured two presumably independent traits (exploratory and risk-taking behaviour) and found that besides the phenotypic correlation, these traits were strongly genetically correlated (chapter 6).
In this thesis we have shown that (i) personality traits have a clear genetic basis, that (ii) the structure of inheritance is not simply additive and that (iii) personality traits do not inherit independently of each other, and that (iv) therefore the genetic structure has to be taken into account when looking at the expected response to natural selection and past evolutionary forces. This has brought us a large step further in understanding the inheritance of complex behavioural traits in natural populations and will help building more realistic models in studying the evolution of complex traits and syndromes of traits. Moreover, with this study we provide the starting point for future research on more detailed questions on several levels, however without ignoring the development in others
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