Laccetti, Lucrezia (2025) The role of local adaptation and habitat fragmentation in driving Mediterranean plant evolutionary trajectories. [Tesi di dottorato]

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Abstract

Geographic variation in plant traits is usually assumed to be the consequence of local adaptation to biotic and abiotic selective pressures, a process of fundamental importance in evolutionary and conservation biology. Local adaptation is more likely to occur within species whose distribution and dispersal are constrained by natural barriers, hence limiting the homogenizing effect of gene flow. In fragmented populations, local adaptation can drive key evolutionary processes eventually leading to species diversification. Despite this evidence, little is known about local adaptation and its environmental drivers in Mediterranean cliff plant species which inhabit environments characterized by strong natural barriers with elevated endemism. In the present thesis, to fill this knowledge gap, I focused on two Mediterranean cliff species with a highly fragmented distribution: Dianthus rupicola and Brassica incana. Specifically, the thesis aims to investigate the drivers and the potential evolutionary implications of local adaptation in fragmented populations and is articulated in three different sections: (1) local adaptation to biotic and abiotic factors (Section I), (2) adaptation to climate changes and ecological resilience (Section II), and (3) early insurgence of reproductive isolation (Section III). In Section I, I investigated patterns of local adaptation in the early phases of plant development in multiple populations of B. incana and their influence on the environmental-dependent pressures experienced by plants in later life stages; in Section II, I assessed how habitat fragmentation and local adaptation can shape plant response to novel and potentially challenging environments both in D. rupicola and B. incana populations; in Section III, I tested for the presence of early insurgence of reproductive isolation between two recently diverged ecotypes of D. rupicola. The results obtained in this thesis highlight, first of all, how local adaptation can be particularly strong in the early phases of plant development and that this strong adaptation has also the potential to shape species evolutionary trajectories by influencing traits expressed by plants in later life stages. Further, this thesis shows evidence that, in fragmented habitats, local adaptation is tightly linked with plant response to novel environments predicted under climate change. Lastly, it demonstrates how geographic fragmentation and diverging biotic selective pressures can boost species diversification in the Mediterranean due to the rapid insurgence of reproductive barriers. However, it is also shown that this process might be reversed in secondary contact zones due to the presence of incomplete reproductive isolation. This finding points out the risks for biodiversity under a scenario of species distribution displacement, which is considered one of the most likely consequences of climate change. Overall, the results obtained in this thesis highlight how in two different species the fragmented cliff habitat can deeply affect their adaptive and evolutionary processes. In doing so, this thesis contributes to a broader understanding on the drivers and consequences of local adaptation in fragmented populations, also providing crucial insights for predicting species evolutionary trajectories under a climate change scenario.

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