Evolutionary genetics of life-history traits in a structured environment : Understanding variation in clutch size and laying date in great tits (Parus major)

Abstract

Understanding variation in clutch size and laying date in great tits (Parus major) Some of the key questions evolutionary biology aims to answer are how genetic variation is being maintained in traits that are closely related to fitness, how populations are adapted to their environment, and, if their environment changes, how they are able to cope with these changes. Especially the question of how well natural populations can cope with rapid environmental changes is becoming increasingly important given that a continuously increasing human population accompanied by a rapid expansion of human activities is resulting in unprecedented rates of environmental change.

Answering these fundamental questions requires the quantification of not only the genetic variances (or heritabilities) and covariances (or genetic correlations) for a suite of traits, but also of the selection pressures acting upon these traits. In this thesis I deal with the evolutionary genetics of natural populations, and thus with genetic variation, selection, and the interaction between the two. More specifically, I investigate how genetic variation and selection are shaped by their environmental and structural context. The goal of this thesis is to obtain a better insight into the evolutionary quantitative genetics of wild populations, with special reference to the role of environmental variation within and among years, as well as within and among populations. I discuss different methodological approaches to deal with such environmental structuring, and I apply these to great tit clutch size and laying date, using both long-term and experimental data. I investigate how both genetic means and variances differ among populations, and how such differences (or their absence) can be explained in the light of gene flow and selection. I provide empirical evidence that population structure, and thereby environmental and genetic structure, may exist on a small spatial scale, and that, when such structure is overlooked, this may have large effects on estimates of quantitative genetic parameters and selection. However, provided we employ the appropriate methods and take into account the interplay between the different processes, then estimates of genetic variation and selection can provide us with valuable insights into both the direction and the potential rate of life-history evolution in natural populations, and thus into their capacity to cope with both random and directional environmental change.

Spatial and temporal variation at both the individual and the population level in major life-history traits like clutch size and laying date are shaped by genes and the environment, and by selection and gene flow. Although these can all be studied independently from each other, I argue here that it is in particular the interplay between genetic and environmental variation, and between population structure, gene flow and selection that is shaping genetic patterns across time and space. We can therefore only understand variation in clutch size and laying date, and life-history traits in general, if on the one hand we take into account small scale genetic and environmental variation within single populations, and if on the other hand we do not treat these populations as independent units that are isolated from the rest of the world.