Le vendredi 20 février 2026,  Thibault KASPRZYK présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Biologie des organismes et écologie) sous la direction d'Alain VANDERPOORTEN.

Cette épreuve consistera en la défense publique d’une dissertation intitulée :

« Inferring mechanisms of community assembly from phylogenetic diversity ».

Le Jury sera composé de :

M. F. COLLART (Président), Mme et MM. D. BAURAIN, B. FREDERICH (Secrétaire), O. HARDY (ULB), S. PAVOINE (Sorbonne Université), A. VANDERPOORTEN (Promoteur).

Abstract

The distribution of species is the result of past and present processes that shape the assemblage of their communities. Niche differentiation and competition are the two main ingredients of a trade-off between selection of traits for a given environment, enhancing fitness in the latter, and competition among closely related species. Darwin was one of the first to hypothesize a connection between niche differentiation and competition and species relatedness, offering an appealing framework to disentangle community assembly processes based on phylogenetic diversity patterns. This framework was later formalized by Chesson (2000) who explains species coexistence by two types of fundamental processes. Equalizing processes minimize fitness differences between species, so that coexisting species tend to share similar functional traits in a given habitat. In turn, stabilizing processes stabilize coexistence via negative density dependent selection, so that coexisting species tend to have dissimilar functional traits to avoid competition. Equalizing processes are thus expected, if adaptive traits are phylogenetically heritable, to generate communities with species more phylogenetically related to each other than expected by chance, a pattern known as phylogenetic clustering. Stabilizing processes, conversely, lead to the assemblage of communities with species less phylogenetically related to each other than expected by chance, a pattern known as phylogenetic overdispersion. The signature that assembly mechanisms leave in community phylogenetic structure has been used to infer community assembly mechanisms from patterns of phylogenetic diversity. Community assembly is, however, the result of a mixture of several processes, including potentially confounding factors associated with dispersal limitations and spatial effects, casting doubt about the application of phylogenetic diversity metrics to infer community assembly processes. Here, we re-assess the extent to which phylogenetic diversity can indeed be used as a proxy for mechanisms of community assembly. We implemented a novel, highly parametrizable and customizable spatially explicit model involving limited dispersal, drift, trait-based selection, and competition, to simulate community assembly under competing processes in a landscape with contrasted habitat connectivity. We subsequently implemented this approach to infer the evolutionary mechanisms underlaying one of the most pervasive biodiversity patterns, namely the latitudinal diversity gradient, using liverworts, a group of early land plants comprised of about 7000 species, as a model.

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