An experimental test of the role of selection, drift and gene flow in shaping evolution in the wild

Despite being at the heart of evolutionary biology, we still have a remarkably poor understanding of the capacity of wild populations to adapt to changing environments. In particular, we struggle to understand why evolution-in-action appears to be so rare, despite genetic variation and natural selection - its main requirements - seemingly being so common. This so-called 'paradox of stasis' raises the question if we need (even) better theory and (even) more data, or if evolution in the wild is in fact inherently unpredictable. This project aims to answer this fundamental question by capitalising on over 60 years of individual-based data for an island population of great tits and a unique eight-year experiment in which we applied strong artificial selection on clutch size, thereby experimentally displacing this population from its adaptive optimum.

Almost two decades after this experiment ended, we are now, for the first time, in the position to investigate what happened when artificial selection ceased: Did natural selection cause clutch size to evolve back to its pre-experimental optimum? If it did, is the speed at which it evolved back in line with our expectations? And if it is not, is this because of the consequences of the other two main contemporary evolutionary forces, gene flow and genetic drift? These non-adaptive drivers of evolutionary change may play a crucial role in constraining adaptation, especially in small and isolated populations, thereby hampering our ability to predict evolutionary change in nature. But as of yet, we lack a comprehensive understanding of how they interact with natural selection to shape evolution in the wild.

To answer these questions, we propose to capitalise on the opportunities provided by this unique experiment by sequencing the genomes of over a thousand birds and combining these data with detailed individual reproductive histories and multi-generational family trees, many of which go back all the way to the start of the long-term monitoring program in the 1950's. Combining these two complementary resources will provide us with an exceptional opportunity to achieve two distinct but closely interrelated objectives.

First, we will test for evolution-in-action in response to natural selection created by our artificial selection experiment. This will provide an experimental test for adaptive evolution in the wild. Furthermore, by using and adapting a complementary set of state-of-the-art analytical approaches, we will be able develop guidelines for future studies of the evolutionary dynamics of other wild populations.

Second, we will ask if and how evolutionary (i.e., genetic) changes in mean clutch size during the aftermath of the artificial selection has been shaped by (adaptive) natural selection, as well as (non-adaptive) random genetic drift, and gene flow mediated by immigration from the mainland and dispersal within the island.

By testing fundamental evolutionary models, this project has the potential to significantly alter our understanding of the adaptive potential of natural populations and will bring us closer to answering some of the longest-standing questions in evolutionary biology. In doing so, we will assess and refine existing, and develop new, analytical tools. We will promote their application to other populations, not only because it will provide an insight into the generality of our findings, but also because they will be able to inform the genetic management of wild populations aimed at maintaining genetic diversity and preventing inadvertent selection and/or evolution. Finally, by providing a textbook example of evolution-in-action, and its drivers, this project provides an exceptional opportunity to communicate key biological concepts to the general public.