Causes and consequences of population fragmentation

Species live in an environment which varies both in time and space. Yet, most of the theory concerning evolutionary adaptation of species to temporally changing environments ignores spatial structure. This is both because the problems are harder to solve formally and because often, it is thought that the spatial structure merely obscures our understanding of important processes in evolution. However, spatial dimension must be included when studying how do the species ranges change through time. In response to the environment, species ranges can shift through space, expand, contract or fragment. Recently, I have discovered that a spatial distribution of a species may suddenly fragment even if the conditions change smoothly through time. This is as yet unstudied but potentially very important phenomenon. A spatial fragmentation throughout a species range or near its margin is commonly observed but it is typically just attributed to sedentary nature of the focal species or its patchily distributed host. Yet, I have shown that such a fragmentation can arise dynamically, from a failure to adapt to continuously changing conditions. It leads to an abrupt loss of species genetic diversity, and dramatically reduces the total population size. Therefore, range fragmentation can substantially increase the likelihood of a species extinction. Moreover, the faster the conditions change through time, the more likely such a range fragmentation is. This can have profound consequences on biodiversity, which is facing escalating rates of environmental change. Using a combination of analytical and numerical approaches, I propose to first disentangle the relative importance of the many factors driving the complex evolutionary and population dynamics of a single species. The main aim is to develop a general, robust theory of species range dynamics, which can then be tested in nature. By determining the driving parameters, we can assess the species vulnerability, and suggest appropriate strategies to increase their resilience in the light of changing environmental conditions.

There is an urgent need for realistic predictions of evolutionary and ecological responses to changing environments, particularly in times of accelerating climate change. Current predictions, however, rely on theory that neglects important eco-evolutionary feedback, such as the effects of declining population sizes and loss of genetic variation, limiting the utility of these predictions. Achieving the realism we need requires a predictive theory that incorporates the feedback between ecology and evolution, while accounting for changes in genetic variance due to selection, mutation, dispersal and genetic drift. In the core paper of this project, I have shown that incorporating this eco-evolutionary feedback fundamentally alters the theoretical predictions. In particular, selection, genetic drift and population size dynamics can act together to lead to a tipping point, where genetic drift suddenly overwhelms selection. Beyond this threshold, adaptive genetic variance becomes depleted, and the species' range contracts or fragments. Upon fragmentation, the species' ability to adapt to further environmental change is severely compromised due to depleted variance and increased demographic instability, setting the stage for future extinction. This theory establishes theoretical foundations for integrating evolution with population structure and dynamics, emphasising the role of local dispersal in maintaining adaptive genetic variance and counteracting genetic drift. The core parameters identified by this theory are measurable, opening avenues for quantitative tests of the theory as well as real-world applications such as species' resilience in a changing climate.